Investigating the particle precipitation of a moving cusp aurora using simultaneous observations from the ground and space
© Taguchi et al.; licensee Springer. 2015
Received: 13 October 2014
Accepted: 25 April 2015
Published: 16 May 2015
Using observations of a moving cusp aurora from a high-sensitivity all-sky imager at Longyearbyen, Svalbard, and in situ observations of the precipitating particles from a spacecraft that flew over the aurora, we examined the particle precipitation features in the early and final stages of the moving cusp aurora. We focused on two auroral structures created near noon, separated by approximately 3 min, during a southwestward interplanetary magnetic field (IMF) condition on 17 December 2012. The second auroral structure occurred when the IMF turned further southward. Immediately after the appearance of the latter structure, the two auroral structures were adjacently situated, and the DMSP F18 spacecraft passed through these regions. A detailed comparison of the data from particle spectrometers onboard the spacecraft and the 630 nm aurora image data demonstrates that the ion precipitation in the young cusp aurora (i.e., second auroral structure) had a high energy flux, whereas that in the old cusp aurora (i.e., first auroral structure) had a very low energy flux. For the electron precipitation, the features in both regions were found to be very similar; the energy flux at approximately 100 eV often exceeded 1 × 109 eV cm−2 s−1 sr−1 eV−1 in both regions. This indicates that the electron precipitation in the moving cusp aurora is maintained at a high flux level over a certain interval from its starting time. Thus, we suggest that the electron precipitation flux in the moving cusp aurora is controlled by a mechanism independent of the ion precipitation.
KeywordsHigh-latitude ionosphere Aurora Particle precipitation Cusp
A moving mesoscale auroral structure, which is characterized by high red line intensities, is a typical phenomenon in the dayside cusp of the high-latitude ionosphere. Its motion generally has a poleward component and very often also has a strong azimuthal component. As has been shown by many researchers (e.g., Sandholt et al. 1986; Lockwood et al. 1989; Fasel 1995; Sandholt and Farrugia 2003; Oksavik et al. 2004; Taguchi et al. 2012), the azimuthal motion is eastward or westward, which is consistent with the direction of the interplanetary magnetic field (IMF) B Y-related tension force arising from the curvature of the open magnetic field line after reconnection on the dayside magnetopause.
A moving auroral structure that is observed well away from the main red line auroral band in the daytime sector is recognized as being the ionospheric signature of open magnetic flux motion driven by patchy or intermittent reconnection, known as a flux transfer event (FTE) (Haerendel et al. 1978; Russell and Elphic 1978). For a moving auroral structure that appears to be embedded in the red line auroral band, however, it is difficult to determine whether such a structure is the signature of an FTE. However, this auroral structure should also be located in the open magnetic field line region because the open/closed field line boundary generally coincides with the equatorward edge of the red line auroral band (Milan et al. 1999).
Immediately after the open magnetic field line is created by reconnection, magnetosheath plasmas (both electrons and ions) stream into the flux tube. Since the flux tube is moving, the injected particles with higher (or lower) velocities reach the ionosphere after they travel a shorter (or longer) distance. For ions, this behavior is observed over a period of several hundred seconds (Lockwood and Smith 1994) because ion velocities are typically comparable to the velocity of the flux tube. The ion energy flux also tends to decrease with increasing distance from the origin of the open flux tube, as is inferred from the results of the modeling of the particle precipitation for steady-state reconnection by Onsager et al. (1993).
For the electron precipitation in the moving cusp aurora, it is still not understood how the electron precipitation changes with increasing distance from the origin of the open flux tube. Satellite observations immediately inside the magnetopause have demonstrated the motion of flux tubes containing enhanced magnetosheath-like electrons (Owen et al. 2001). Since electrons have high speeds along magnetic field lines, they may come down to the ionosphere in time scales on the order of seconds, suggesting that the electron features of the ionospheric signature of the open flux tube reflect the spatial change in the magnetosheath density as a function of the downstream distance from the reconnection point.
Another possibility is that the precipitating electrons are affected by the ions that are injected with them, rather than simply reflecting the spatial change in the magnetosheath. Burch (1985) suggested that the cusp electrons follow the ions to maintain charge neutrality. If this behavior is operative in the flux tube of the moving cusp aurora and the mechanisms for maintaining the behavior do not change significantly with increasing distance from the origin of the flux tube, a fixed relationship may be seen between the features of the precipitating electrons and those of the precipitating ions, irrespective of the distance from the origin of the open flux tube.
At ionospheric heights, it is difficult to discern the evolution of the moving particle precipitation region using in situ data from a single spacecraft because the speed of low-altitude spacecraft is considerably greater than the speed of the precipitation region. The data obtained by a spacecraft would simply reflect the spatial structure inside the region without providing any information on its evolution. One powerful tool that does provide information on the evolution of the precipitation region is the all-sky imager. An all-sky imager can be used to understand when and where auroral structures are created and how they subsequently move. If an all-sky imager observes multiple moving auroral structures whose elapsed times from their appearance differ and a spacecraft flies over these structures, then the spacecraft data can provide information on how the precipitation region changes with elapsed time from the appearance of the structures or with increasing distance from their points of origin.
For a single moving auroral structure, Oksavik et al. (2005) presented a simultaneous observation event of this type. Data from the ion drift meter on the Defense Meteorological Satellite Program (DMSP) F13 spacecraft for this event indicated the presence of mesoscale convection for a moving cusp aurora, whereas the precipitating particle data from this satellite did not show any prominent signatures. This latter result was presumably due to the fact that the satellite observation was made after the aurora disappeared (10:12:00 UT in Figure four of Oksavik et al. 2005). A moving cusp aurora has a short lifetime, typically 3 to 4 min (Fasel 1995). This short lifetime also makes it rare for their presence to coincide with the measurements of in situ spacecraft.
In this paper, we report results obtained on 17 December 2012 from the simultaneous observation of two cusp auroral structures by an all-sky imager and a spacecraft. We show the motion of the two auroral structures near local noon using the 630 nm auroral image captured by the all-sky imager. One auroral structure arrived at the satellite position approximately 230 s after its initial appearance (although the height of the aurora was much lower than the altitude of the satellite), whereas the other arrived at a similar position only 60 s after its initial appearance. We present observations of the precipitating particles made by the DMSP F18 spacecraft that flew over these auroral structures and show the precipitation features of the ions and electrons in the young and old flux tubes (i.e., the flux tubes of the auroral structures that appeared second and first, respectively).
The instrumentation used in this study consists of a ground-based all-sky imager and particle spectrometers on the DMSP F18 spacecraft, which flew at an altitude of approximately 840 km. The all-sky imager uses an electron multiplier charge-coupled device (EMCCD) camera (Hamamatsu, C9100-13, Hamamatsu, Japan) with an imaging resolution of 512 × 512 pixels and measures emission at two wavelengths, 557.7 and 630.0 nm, using narrow passband interference filters. This ground-based imager has been operating in Longyearbyen, Norway, (geographical latitude 78.1° N and longitude 16.0° E) since October 2011 (Taguchi et al. 2012). As shown in Taguchi et al. (2012), the 630.0 nm (red) line data from this EMCCD camera can provide detailed information about the dynamic features of moving cusp auroral structures. The high sensitivity of this camera is also very effective for identifying the structure of polar cap patches (Hosokawa et al. 2013a, 2013b, 2014; Sakai et al. 2014). Thus, in the current study, we used 630.0 nm wavelength images.
In the first observation season, from the end of October 2011 to the end of February 2012, we obtained 630.0 nm data with an exposure time of 4 s per image. For the second year of observation, which includes the event studied in this paper, we modified the observation mode slightly so that the 630.0 nm images could be obtained with an exposure time of 1 s in addition to the exposure time of 4 s. For this study, we used the imaging data with an exposure time of 1 s. As is demonstrated later, images with this short exposure time were more helpful for identifying the location of auroral brightening, even though the data were not obtained at regular intervals, because of the constraints of the software controlling the automated observation system.
The particle spectrometers on the DMSP F18 satellite measure precipitating electrons and ions with energies between 30 eV and 30 keV once per second (Hardy et al. 1984). Since electron precipitation with an energy of approximately 100 eV strongly affects the 630.0 nm emission rate (e.g., Roble and Rees 1977), out of the full available energy range of the spectrometer, we concentrated on the electron data from three energy intervals near 100 eV, specifically at approximately 70, 100, and 150 eV.
Observations of the cusp aurora on 17 December 2012
where X SC and Y SC represent the spacecraft position in the GSE coordinate system, X MP is the magnetopause location, B X and B Y are the X- and Y-components of the IMF in the GSE coordinate system, and V SW is the solar wind speed. The second term on the right-hand side, 1.33 X MP, represents an approximation of the subsolar standoff distance of the bow shock (Fairfield 1971). The factor of 2.64 in the fourth term comes from the multiplication of 0.33 and 8, where 8 is the slowdown factor of the solar wind in the subsolar magnetosheath (Spreiter and Stahara 1980; Khan and Cowley 1999). The last term, 2 min, represents the Alfven wave travel time from the magnetopause to the ionosphere. We assumed X MP to be 10 R E and obtained T delay = −7 min by using (X SC, Y SC) = (23.5, 17.8) R E and (B X, B Y) = (−5.2, −1.3) nT as average values for 08:20 to 08:30 UT and taking V SW to be 410 km s−1 from the ACE data.
The ACE data (black line) plotted in Figure 2 are 16 and 64 s averages of IMF and plasma data, respectively. A delay of 47 min was introduced in the ACE data to align the sharp increase in the ACE B Z with a similar increase in the GEOTAIL B Z at approximately 08:45 UT. The two dotted lines indicate the time at which the aurora images of Auroras B and C (Figure 1) were first captured, specifically 08:22:28 and 08:25:33 UT, respectively. The aurora images will be shown later. As is shown in both spacecraft data, the IMF B Z was very stable at approximately −5 nT (Figure 2c) during the interval of approximately 1 h before the appearance of the auroral structures.
The GEOTAIL IMF data (red line) indicate that Aurora B appeared when (B Y, B Z) was approximately (−4, −6) nT. Immediately following this, the IMF turned further southward, and when (B Y, B Z) was approximately (−2, −7) nT, Aurora C appeared. As is shown later, the motion of Aurora C had a smaller eastward component than that of Aurora B, which is consistent with these IMF values.
The X-component B X of the GEOTAIL IMF was relatively constant during the interval of approximately 1 h before the appearance of the auroral structures of interest (Figure 2a). A negative B X when B Z < 0 may be a relatively favorable condition for stronger auroral emission on the dayside (Yang et al. 2013). The solar wind plasma density N sw was approximately 5 cm−3 during that interval (Figure 2d). The solar wind speed V sw and the dynamic pressure P D were also stable at approximately 410 km s−1 (Figure 2e) and in the range of approximately 1 to 2 nPa (Figure 2f), respectively.
Particle observations from the DMSP spacecraft
At approximately 08:25:15 UT, the spacecraft entered the ion precipitation region that is typical for the cusp. Specifically, precipitating ions with energies between 100 eV and 1 keV became evident at 08:25:18 UT, i.e., 75.50° magnetic latitude (MLAT) and 13:33 MLT. The total energy flux and average energy of the ions generally increased with decreasing latitude (first and second panels, red dots) until 08:27 UT. After that, the energy flux became somewhat low (top panel, red dots). After 08:27:14 UT, the ion energy flux in the range of 1 to 10 keV (fourth panel) was lower than 1 × 106 eV cm−2 s−1 sr−1 eV−1 (becoming a greenish color). We determined this latitude (72.54° MLAT) to be the equatorward boundary of the cusp. The cusp range is represented by the red horizontal bar between the two spectrograms. Presumably, the region equatorward of this range is on a closed field line because the energy flux of the precipitating electrons at approximately 100 eV (i.e., electrons of magnetosheath origin) is very low. As will be shown in the following section, the most notable result is that the cusp includes the region containing precipitating electrons with a higher energy flux (reddish color in the third panel).
Results and discussion
Multiple boundary crossings inside the cusp
Figure 4 shows the boundaries between different precipitating plasma features. Boundary 1, which is the leftmost line of the six lines on the plot, is the position over which the DMSP satellite flew at 08:25:25 UT. In the electron spectrogram (Figure 4b), this boundary represents the point where the maximum energy flux for the three energy steps around 100 eV first exceeded 1 × 108 eV cm−2 s−1 sr−1 eV−1 (yellow color) for two consecutive seconds. This point occurred at (MLAT, MLT) = (75.4°, 13:26). These two consecutive data bins, i.e., the observations over the given 2 s, corresponded to a horizontal scale of approximately 15 km along the satellite path. An aurora of this horizontal size would be detected over an area larger than 2 × 2 pixels (out of 512 × 512 pixels) wherever the aurora is situated inside the imager’s field of view. The 2 × 2 pixel area is the smallest that can be reasonably resolved. Boundary 1 was very close to the poleward boundary of the cusp ion precipitation (08:25:18 UT), which was defined using the ion precipitation in Figure 3.
Boundary 2 is located at the position the DMSP satellite crossed at 08:26:14 UT; at this position, (MLAT, MLT) = (74.37°, 12:44). This boundary was determined from the sharp drop in the integral number flux ratio (Figure 4a). This can also be seen as a sharp drop in the ion energy, as at this time, the energy of the peak ion energy flux dropped to approximately 200 eV with a decrease in the ion energy flux (Figure 4c). After this time, low-energy ion precipitation appeared intermittently. Just 1 s later, i.e., at 08:26:15 UT, the satellite entered the region where the energy flux of the electron precipitation was very high; the maximum of the energy flux for the three energy bins around 100 eV exceeded 1 × 109 eV cm−2 s−1 sr−1 eV−1. We refer to the region before Boundary 2 and after Boundary 1 as Region a.
The next boundary, Boundary 3, was chosen as the position the DMSP satellite crossed at 08:26:26 UT; at Boundary 3, (MLAT, MLT) = (74.05°, 12:35). At this time, there was a significant jump in the integral number flux ratio (Figure 4a). The maximum electron energy flux from the three energy bins around 100 eV decreased to values below 1 × 108 eV cm−2 s−1 sr−1 eV−1, similar to the values often observed prior to Boundary 2. In addition, at this time, the energy flux of the ion precipitation sharply increased and approached that observed before Boundary 2. We refer to the region between Boundaries 2 and 3 as Region b. Region b is characterized as the region where the integral ion number flux relative to the integral electron number flux is very low compared with Region a.
After 08:26:26 UT (Boundary 3), ion precipitation with features similar to those seen in Region a appeared and continued for a brief period of time. At 08:26:30 UT, the energy of the peak ion energy flux dropped to well below 1 keV. We designated this position Boundary 4. The electron features in the region between Boundaries 3 and 4 are similar to those near the equatorward edge of Region a, i.e., the region observed during 08:26:08 to 08:26:10 UT. As was mentioned above, the electron energy flux dropped sharply at Boundary 3, and that low value is also similar to the values that were often observed prior to Boundary 2. The integral number flux ratios between Boundaries 3 and 4 are also similar to the ratios prior to Boundary 2. We thus regarded the region between Boundaries 3 and 4 as a reappearance of Region a.
For a while after 08:26:30 UT (Boundary 4), the satellite continued to observe high electron energy flux. These electron features are very similar to those seen in Region b. However, there is a clear difference in the ion precipitation features. The energy flux of the ions was higher after Boundary 4 than in Region b. The integral number flux ratio was also higher after Boundary 4 than in Region b. These observations are crucial results of this study and will be used later for comparison with the auroral image. At 08:26:47 UT, the integral number flux ratio increased sharply, and the maximum energy flux of the electron precipitation for the three energy bins around 100 eV no longer reached 1 × 109 eV cm−2 s−1 sr−1 eV−1. We designated this position Boundary 5, and we refer to the region between Boundaries 4 and 5 as Region c.
Although the energy flux of the electron precipitation shows a sharp drop at Boundary 5, this flux remained relatively high for a while longer. Boundary 6 (at 08:26:58 UT) was determined to be the last point of that relatively high-flux period. After this boundary, there were no further instances of the maximum energy flux for the three energy bins with two consecutive values exceeding 1 × 108 eV cm−2 s−1 sr−1 eV−1. At this boundary, (MLAT, MLT) = (73.08°, 12:12); this latitude was higher than the equatorward boundary of the cusp (72.54°) by approximately 0.5°.
Electron precipitation for each auroral structure
Figure 5b shows the position of Boundary 6 (red square) on the 630.0 nm all-sky image taken at 08:27:06 UT, which was 8 s after the satellite passed through Boundary 6 at 08:26:58 UT. Boundary 6 was located near the equatorward boundary of the 630 nm dayside auroral oval. This provides clear evidence that the cusp aurora, including its equatorward part, is located in the open field line region, though this feature has already been suggested with HF radar and a meridian-scanning photometer by Milan et al. (1999). We checked all auroral images obtained between 08:26:58 and 08:28:48 UT (=08:26:58 + 110 s). The position of Boundary 6 relative to the equatorward boundary of the 630 nm auroral oval was very similar throughout this interval.
Figure 6a shows the positions of Boundaries 2, 3, 4, and 5 on the 630.0 nm all-sky image taken at 08:26:52 UT, which was also shown in Figure 1. This time is 38, 26, 22, and 5 s after the detection of Boundaries 2, 3, 4, and 5, respectively. As indicated in Figure 6a, the region between Boundaries 2 and 3 (Region b in Figure 4) overlapped Aurora B, and the region between Boundaries 4 and 5 (Region c in Figure 4) overlapped the eastward part of Aurora C. As will be shown later, Aurora B was moving eastward, whereas Aurora C did not move substantially at that time. Subsequently obtained images also show that the poleward and equatorward boundaries of Aurora C were Boundaries 4 and 5, respectively. Figure 6b, taken at 08:27:33 UT, is an example of such an image. This image was taken 46 s after the DMSP observation of Boundary 5. At this time, Aurora C was still situated between Boundaries 4 and 5. Note that because the excited oxygen atom has a relatively long lifetime (theoretically 110 s, as mentioned above), obtaining a more precise relationship between the extent of the moving auroral structure and the electron precipitation region would require information about how the neutral wind, with which the excited oxygen atom moves, was distributed during that period of time.
Change in the positions of Auroras B and C
As shown by the black arrow in each of the subsequent images, this auroral structure moved slightly poleward until 08:25:53 UT (Figure 8i) and then accelerated strongly eastward, which is consistent with the motion of the reconnected field line for a negative IMF B Y (Figure 2). The image shown in Figure 8k is from the same data source as those shown in Figures 1 and 6a. The location of Aurora B at 08:27:19 UT (Figure 8l) is (MLAT, MLT) = (74.5°, 12:24). We determined this location from the trailing edge of the aurora whose emission is more than 4 kR. The DMSP observation of Region b (i.e., 08:26:14 UT for Boundary 2) was obtained in the middle of the eastward motion of the aurora and approximately 230 s after the appearance of Aurora B. After 08:27:19 UT (Figure 8l), Aurora B was no longer evident, and thus the observation by the spacecraft at 08:26:14 UT was obtained just before Aurora B disappeared, i.e., in the final stage of the aurora.
The motion of Aurora B suggests that it takes a few minutes (from 08:22:28 to 08:25:53 UT) for the result of the tension exerted by the reconnected field line to become prominent in the ionosphere after the appearance of the cusp aurora. A similar tendency can be identified in a larger eastward-moving auroral structure, which was seen over the mainland of the Svalbard Islands (Figure 8f,g,h,i,j,k). During the interval shown in Figure 8f,g,h,i,j,k, the auroral structure was moving eastward, but prior to that (Figure 8d,e), the auroral structure had been moving poleward, roughly along the 1100 MLT meridian. A recent study based on simultaneous observations from the Super Dual Auroral Radar Network (SuperDARN), a far-ultraviolet instrument on the IMAGE spacecraft, and a ground magnetometer also reported similar flow features for the cusp proton aurora event (Taguchi et al. 2010).
The interval between the appearance of Aurora C and the DMSP observation (60 s) is short when compared with the interval for Aurora B, which was approximately 230 s. We have shown that the energy flux of the ion precipitation in Aurora C was much higher than that in Aurora B. We know that the energy flux of the precipitating ions in the young open flux tube (i.e., Aurora C) was much higher than in the old open flux tube (i.e., Aurora B). This is consistent with the signatures expected from reconnection, indicating that the energy flux of the ion precipitation tends to decrease with increasing distance from the footprint of the reconnection point (e.g., Onsager et al. 1993).
High-flux electron precipitation
The high fluxes of the electron precipitation in both Regions b and c suggest that the energy fluxes of the precipitating electrons in the young and old open flux tubes do not differ significantly. For the present event, the maximum differential energy flux at approximately 100 eV similarly exceeded 1 × 109 eV cm−2 s−1 sr−1 eV−1 in both regions. In contrast, as mentioned above, the energy fluxes of the precipitating ions in those regions differed significantly. Thus, it appears that the electron precipitation flux level is controlled by a mechanism independent of the ion precipitation flux.
Further studies are needed to understand the process that directly produces the high electron precipitation flux inside an open flux tube; however, it is estimated that an upward field-aligned current whose carrier is the precipitating electrons and a parallel potential drop, which is presumably produced in conjunction with the upward field-aligned current, may be important factors. A mesoscale convection system consisting of upward and downward field-aligned currents was proposed for the ionospheric signatures of the FTE by Southwood (1987); subsequently, Oksavik et al. (2004) showed that such a flow pattern actually exists. Collocation of the mesoscale upward field-aligned current with intense electron precipitation can be seen in an event reported by Taguchi et al. (1993) (e.g., 02:51:32 to 02:51:40 UT in Plate 1 of that work). Using simultaneous satellite observations of electrons at two altitudes in the cusp, Lin et al. (1986) showed that some parallel potential drop is needed to explain the enhanced energy flux observed in the lower-altitude electrons, although we do not know whether their enhanced energy flux events are related to cusp moving auroral structures.
In this study, we demonstrated a one-to-one correspondence between the moving cusp auroral structure measured at a wavelength of 630.0 nm and the electron precipitation region responsible for its creation. This result will help develop the quantitative relation between the red line aurora intensity and soft precipitating electrons. In Regions b and c, the integral electron energy flux increased to 2 × 1011 to 4 × 1011 eV cm−2 s−1 sr−1, and the average energies at those times were approximately 2 × 102 eV (see the black dots in the top two panels of Figure 3 for the corresponding interval). From the intensities of Auroras B and C, we know that the electron precipitation with these characteristics created a 4.5 to 5 kR red line emission.
Using data from the high-sensitivity all-sky imager at Longyearbyen, Svalbard, and particle data from the DMSP F18 spacecraft, we examined particle precipitation features in the early and final stages of a moving cusp auroral structure. We focused on two structures that occurred near noon, separated by approximately 3 min, during a southwestward IMF condition. The second structure occurred when the IMF turned further southward. Immediately after the appearance of the latter structure, the two auroral structures were adjacently situated, and the DMSP F18 spacecraft passed through these regions.
To delineate the boundaries of different precipitating regions in the DMSP particle data, we introduced the ratio of the integral ion number flux to the integral electron number flux. Our investigation shows that the ion precipitation in the young cusp aurora had a high energy flux, while that in the old aurora had a very low energy flux. Regarding the electron precipitation, the features in both regions were found to be very similar: the energy fluxes at approximately 100 eV often exceeded 1 × 109 eV cm−2 s−1 sr−1 eV−1 in both regions. From this study, we suggest that the precipitation of the electrons contained in the open flux tube is maintained at a high flux over a certain period of time from its beginning and the high flux of the electron precipitation is controlled by a mechanism independent of the ion precipitation.
Advanced Composition Explorer
Defense Meteorological Satellite Program
electron multiplier charge-coupled device
flux transfer event
geocentric solar ecliptic
geocentric solar magnetospheric
interplanetary magnetic field
magnetic local time
Super Dual Auroral Radar Network
We would like to thank Takeshi Aoki of the University of Electro-Communications for his maintenance of the all-sky imager. We also thank Fred Sigernes and Margit Dyrland for their help at the University Centre in Svalbard, Longyearbyen. Discussion with Dai Hasegawa of the University of Electro-Communications is acknowledged. The DMSP particle detectors were designed by Dave Hardy of AFRL, and the data were obtained from JHU/APL. GEOTAIL magnetic field data were provided by T. Nagai through DARTS at the Institute of Space and Astronautical Science, JAXA, in Japan. ACE magnetic field and plasma data were provided by N. Ness and D. McComas through the NASA CDAWeb. This work was supported by JSPS KAKENHI Grant Number 22340143.
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