Statistics of a parallel Poynting vector in the auroral zone as a function of altitude using Polar EFI and MFE data and Astrid-2 EMMA data

Abstract

Abstract. We study the wave-related (AC) and static (DC) parallel Poynting vector (Poynting energy flux) as a function of altitude in auroral field lines using Polar EFI and MFE data. The study is statistical and contains 5 years of data in the altitude range 5000-30 000 km. We verify the low altitude part of the results by comparison with earlier Astrid-2 EMMA Poynting vector statistics at 1000 km altitude. The EMMA data are also used to statistically compensate the Polar results for the missing zonal electric field component. We compare the Poynting vector with previous statistical DMSP satellite data concerning the electron precipitation power. We find that the AC Poynting vector (Alfvén-wave related Poynting vector) is statistically not sufficient to power auroral electron precipitation, although it may, for K p >2, power 25-50% of it. The statistical AC Poynting vector also has a stepwise transition at R=4 R E , so that its amplitude increases with increasing altitude. We suggest that this corresponds to Alfvén waves being in Landau resonance with electrons, so that wave-induced electron acceleration takes place at this altitude range, which was earlier named the Alfvén Resonosphere (ARS). The DC Poynting vector is ∼3 times larger than electron precipitation and corresponds mainly to ionospheric Joule heating. In the morning sector (02:00-06:00 MLT) we find that the DC Poynting vector has a nontrivial altitude profile such that it decreases by a factor of ∼2 when moving upward from 3 to 4 R E radial distance. In other nightside MLT sectors the altitude profile is more uniform. The morning sector nontrivial altitude profile may be due to divergence of the perpendicular Poynting vector field at R=3−4 R E .

High‐Latitude Neutral Mass Density Maxima

“…In our analysis of DMSP DC Poynting fluxes, we have found that the level of electromagnetic energy flux during quiet periods is near zero. We suggest that AC Poynting flux, which is normally a much smaller component of the electromagnetic energy input in the auroral zone (Janhunen et al, 2005), may be related to the quiet time neutral mass density maxima in our study. We are unable to test this hypothesis using DMSP data because there are no wave sensors on the spacecraft.…”

ULF wave electromagnetic energy flux into the ionosphere: Joule heating implications

“…Finally, we note that in a similar study, Janhunen et al [] found that the “AC” S ∥ i was significantly smaller than the “DC” rate, but their study was focused on the auroral acceleration implications of S ∥ i (rather than Joule heating) and had several key differences from this one: (1) only periods when the satellite was conjugate to the nominal auroral oval were included (18–6 MLT, 65–74 invariant latitude), (2) there was no restriction to closed field line regions, (3) use of a satellite with a high‐inclination orbit (Polar), (4) the “AC” definition differs from this study and refers to three different categories of measurements corresponding to different time domain filter cutoffs (two of which roughly correspond to the ULF wave frequencies examined in the present study). For these reasons, some of their results differ substantially from this study; in particular, the spatial patterns found for AC Joule heating in Figure 6 of Janhunen et al [] are different than in Figure in this study. Nevertheless, their overall conclusion that AC S ∥ i is smaller than DC S ∥ i is consistent with the results of this study for nominal conditions, and the typical values they found for S ∥ i are in agreement with those found in this study (Table ).…”

Alfvén Waves and Their Roles in the Dynamics of the Earth’s Magnetotail: A Review

“…However, the studies can not address questions relating to the position of acceleration along the field line, except to point out that acceleration must have occurred somewhere between Polar and FAST. This question was addressed in a series of statistical studies covering the altitude range from 1 to 6 R E (Janhunen et al, 2004(Janhunen et al, , 2005(Janhunen et al, , 2006. As seen in the left section of Fig.…”