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Remote Sensing

Satellite images confirm that polar rain electrons produce auroras

Solar wind electrons can enter the polar atmosphere and create auroras that move from dayside to nightside and show a dawn-dusk aligned structure.
26 January 2008, SPIE Newsroom. DOI: 10.1117/2.1200801.0996

A subset of the solar wind electrons streaming out from the Sun, polar rain electrons1 directly enter the upper atmosphere through open magnetic field lines. With energy flux from 0.001 0.1 ergs/cm2/s, they are usually too weak1 to create the familiar visual aurora. However, when intense (measuring a few ergs/cm2/s flux with mean energy ≥ 1 keV), they sometimes remain active for hours in the polar cap, the poleward region of the auroral oval.2 Although these electrons might be expected to create auroras, until now none had ever been detected or imaged.

Weak diffuse optical emissions in the polar cap during a southward interplanetary magnetic field (IMF) are not traditionally considered auroras. This interpretation largely owes to the difficulty in distinguishing auroral from other atmospheric emissions using ground-based instruments with steeply limited fields of view. However, satellite-based imagers can continuously monitor the global auroral oval and should be helpful for identifying auroras in the polar cap.

Events depict a new type

The polar rain aurora (PRA), a new type, is shown in the images that comprise Figure 1. They were obtained from the SI-13 wavelength channel of the spectrographic imager (SI)3 onboard the IMAGE satellite. A small polar cap without noticeable auroras was seen between 18:39 and 18:52 UT. By 19:13 UT, the dayside cap was filled with auroras that moved in a top-to-bottom (anti-sunward) direction at a speed of ∼720km/hour, becoming a right-left (dawn-dusk) aligned patch through 20:37 UT. Such an aurora was also observed by Defense Meteorological Satellites Program (DMSP) special sensor ultraviolet spectrographic imager (SSUSI)4 in two different channels, as indicated in Figure 2(a) and (b). The particle data5 show that the auroral emissions were due to intense polar rain (keV electrons). Figures 2(c) and (d) provide another example of PRA. Again, it appears as a dawn-dusk aligned auroral patch caused by keV polar rain electrons.5 Sudden enhancement in the 1-10 keV electron flux around 22:00 UT (see Figure 3) gives direct evidence of a solar wind source for the keV polar rain electrons.

Figure 1. Images of the southern hemisphere from IMAGE SI-13.

Figure 2. Auroral images (southern hemisphere) from the special sensor ultraviolet spectrographic imager (SSUSI): (a) and (b) on July 22, 2004; (c) and (d) on September 13, 2004.

Figure 3. Solar wind electron spectra from Geotail satellite.

IMF conditions

For both events5 the associated IMF was southward and dominated by By. However, Bx and By were oppositely orientated for the two events. The PRA and keV electrons were observed only in the southern hemisphere. Such results run counter to the idea6 that IMF Bx orientation determines polar rain entry to the northern or southern hemisphere.

Why do PRAs appear as dawn-dusk aligned patches and move anti-sunward? Three possible factors determine this behavior. First, the dominant IMF By leads to dawn-dusk aligned magnetic flux tubes in the magnetosheath. Secondly, the strong nonoscillatory drift mirror waves7 (NDMWs) modulate the quantity of keV electrons entering the polar atmosphere via the magnetic mirroring force and pitch-angle diffusion by lion roars.7,8 Thirdly, the flux tubes and NDMW drift in an anti-sunward direction.

Summary and future work

The PRA is a newly observed phenomenon due to enhanced keV electrons in the solar wind under a southward IMF. It begins in the dayside polar cap and,while moving anti-sunward, becomes a dawn-dusk aligned patch due to dominant IMF By. A statistical analysis of many events will be required to establish a firm relationship between IMF conditions and the aurora morphology.

Images in Figures 1-3 are reproduced from Zhang et al.5

Yongliang Zhang, Larry Paxton 
Space Department
Johns Hopkins University Applied Physics Laboratory
Laurel, MD

Yongliang Zhang, PhD, is a senior scientist in the Atmospheric Remote Sensing Section in the Johns Hopkins University Applied Physics Laboratory. His research interest is in auroral response to solar wind/IMF changes, aurora effect on neutral atmosphere and ionosphere, and identification/application of non-traditional auroras, such as ring current auroras.

Larry J. Paxton, PhD, is supervisor of the Atmospheric Remote Sensing Section inthe Johns Hopkins University Applied Physics Laboratory. He is the co-principal investigator for the global ultraviolet imager on the NASA thermosphere ionosphere mesosphere energetics and dynamics (TIMED) Imager and the principal investigator on the DMSP special sensor ultraviolet spectrographic imager. He has published over 175 papers on planetary and space science, instruments, remote sensing techniques, and space mission design. He has been a member of SPIE since 1988.