Photoelectron flow and field-aligned potential drop in the polar wind
Location
Yosemite National Park
Start Date
2-10-2014 6:55 PM
End Date
2-10-2014 7:25 PM
Description
We have statistically examined photoelectron spectra in the polar cap obtained by the electron spectrometer aboard the Fast Auroral SnapshoT (FAST) satellite at about 3800 km altitude during geomagnetically quiet periods. We frequently find counter-streaming photoelectrons of a few tens of electron volts, indicating existence of a fieldaligned potential drop above the altitude of the satellite. The estimated typical magnitude of the field-aligned potential drop above the satellite is ~22 V (July 2002 at solar maximum), which is about a half of that predicted by photoelectron-driven polar wind models with a potential drop at high altitudes. Since this potential drop reflects all electrons with energies below the potential drop, including those below the low energy threshold of the instrument (~4 eV), we can derive net escaping electron number fluxes without uncertainty due to the very low energy component. Under small field-aligned current conditions, the net escaping electron number flux should be nearly equal to the number flux of ions. Thus, the existence of the potential drop enables us to estimate the flux of polar wind ions from observations of photoelectrons. It is suggested that this field-aligned potential drop and the reflected photoelectrons at high altitudes would regulate the polar wind system as follows: The net escaping electron number flux negatively correlates with the magnitude of the potential drop; in cases of a large potential drop, most of photoelectrons are reflected and cannot escape. An increase in the magnitude of the potential drop increases reflected photoelectrons that precipitate into the ionosphere. Since these reflected photoelectrons become an additional heat source of the topside ionosphere, they help to develop a stronger ambipolar electric field in a classical way, which would increase the flux of polar wind ions (≈ net escaping electron number flux). Thus, the resulting negative feedback would keep the magnitude of the potential drop relatively stable. The most appropriate balance of this polar wind system (equilibrium state) would be achieved near the median of the magnitude of the potential drop. In contrast to geomagnetically quiet periods, our event studies revealed that the potential drop frequently became smaller than ~5 V during the main and early recovery phases of large geomagnetic storms. During geomagnetic storms, additional ions originating from the cusp/cleft ionosphere convect into the polar cap. These additional ions would change the balance of this polar wind system; the net escaping electron number flux should increase to balance the enhanced ion flux. The magnitude of the potential drop would be reduced to let a larger fraction of photoelectrons escape.
Photoelectron flow and field-aligned potential drop in the polar wind
Yosemite National Park
We have statistically examined photoelectron spectra in the polar cap obtained by the electron spectrometer aboard the Fast Auroral SnapshoT (FAST) satellite at about 3800 km altitude during geomagnetically quiet periods. We frequently find counter-streaming photoelectrons of a few tens of electron volts, indicating existence of a fieldaligned potential drop above the altitude of the satellite. The estimated typical magnitude of the field-aligned potential drop above the satellite is ~22 V (July 2002 at solar maximum), which is about a half of that predicted by photoelectron-driven polar wind models with a potential drop at high altitudes. Since this potential drop reflects all electrons with energies below the potential drop, including those below the low energy threshold of the instrument (~4 eV), we can derive net escaping electron number fluxes without uncertainty due to the very low energy component. Under small field-aligned current conditions, the net escaping electron number flux should be nearly equal to the number flux of ions. Thus, the existence of the potential drop enables us to estimate the flux of polar wind ions from observations of photoelectrons. It is suggested that this field-aligned potential drop and the reflected photoelectrons at high altitudes would regulate the polar wind system as follows: The net escaping electron number flux negatively correlates with the magnitude of the potential drop; in cases of a large potential drop, most of photoelectrons are reflected and cannot escape. An increase in the magnitude of the potential drop increases reflected photoelectrons that precipitate into the ionosphere. Since these reflected photoelectrons become an additional heat source of the topside ionosphere, they help to develop a stronger ambipolar electric field in a classical way, which would increase the flux of polar wind ions (≈ net escaping electron number flux). Thus, the resulting negative feedback would keep the magnitude of the potential drop relatively stable. The most appropriate balance of this polar wind system (equilibrium state) would be achieved near the median of the magnitude of the potential drop. In contrast to geomagnetically quiet periods, our event studies revealed that the potential drop frequently became smaller than ~5 V during the main and early recovery phases of large geomagnetic storms. During geomagnetic storms, additional ions originating from the cusp/cleft ionosphere convect into the polar cap. These additional ions would change the balance of this polar wind system; the net escaping electron number flux should increase to balance the enhanced ion flux. The magnitude of the potential drop would be reduced to let a larger fraction of photoelectrons escape.