Satellite images confirm that polar rain electrons produce auroras

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.

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.

For both events⁵ the associated IMF was southward and dominated by B_y. However, B_x and B_y 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 B_x 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 B_y 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.

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 B_y. 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