Spectral studies of the sources of ionospheric electric fields

Abstract

Spectral analyses have been performed upon a number of incoherent scatter radar data sets obtained at Jicamarca, Peru; Chatanika, Alaska; and Arecibo, Puerto Rico, with the goal of understanding the sources of electric fields with periods in the range of 1–10 hours. Two distinct sources are identified and studied in some detail. In quiet times, atmospheric gravity waves seem the most likely source of the ionospheric electric field. In fact, both in an average sense and in the single case study available the mesospheric winds measured at Poker Flat, Alaska, in this frequency range are remarkably similar in magnitude to the quiet time thermospheric plasma drifts measured overhead by the nearby Chatanika radar. Such drifts are driven by electric fields which, we argue, could easily be generated by the observed wind fields. Comparison with the spectra of electric field measurements at other latitudes suggests that such a source is worldwide and determines the geophysical noise level of low‐ and mid‐latitude electric field measurements. Turning to active times, we present a measure of the transfer function for electric field penetration between high‐ and low‐altitude L shells. At the very lowest frequencies (periods of ≥10 hours) the low‐altitude sites are well shielded, presumably by an Alfvén layer at the inner edge of the ring current. Higher frequency fluctuations penetrate very easily to low latitudes. A response peak seems to occur in the 3‐ to 5‐hour range of periods, with a lower response occurring at 1 cycle/hour, although this result must be viewed as preliminary for now. Between L = 5.5 and L = 1.4 the zonal electric field component as projected to the equatorial plane of the magnetosphere penetrates with little or no attenuation. At Jicamarca the field is reduced to about 75% of the high‐latitude source. Even so, the very high sensitivity of the Jicamarca measurement of zonal electric fields makes this component very easy to detect. We find that for fluctuation periods less than 5 hours the magnetospheric electric field source dominates atmospheric sources for Kp ≥ 3 at Jicamarca. In one remarkable event, six cycles of a 1‐ cycle/hour signal were recorded with nearly identical spectral features in the interplanetary medium, at auroral zone latitudes, and on both sides of the earth at equatorial ionospheric heights. The ratio of the relative magnitude of this oscillation at L = 5.5 to its value at L = 1.04 is in good agreement with the value of the average transfer function at 1 cycle/hour found in the 24‐hour runs.

Letter to the editor: Electric field fluctuations (25-35 min) in the midnight dip equatorial ionosphere

“…In our knowledge, there is only one documented event of similar quasi-periodic electric ®eld¯uctuations of eastward polarity at the nightside (22-04 LT) dip equator. This event of February 17±18, 1976, studied by Gonzales et al (1979) as well as Earle and Kelley (1987), is characterised by¯uctuations with signi®cant power at »1 h period in IMF B z as well as in electric ®elds at auroral and dip equatorial stations. But this event too does not seem to be of DP2 origin because the day-tonight polarity of the equatorial electric ®elds (eastward on nightside and westward on dayside) is at variance with the theoretical pattern of DP2 electric ®elds.…”

Short-Period(5-33 min) Variations in Vertical Drift of F-Region Plasma near the Magnetic Equator.

“…The data thus correspond to bottomside F layer and under non-ESF conditions. The time rate of change of phase path (APIAt) during the evening hours is primarily a measure of vertical plasma motion near the reflection level as the contribution due to electron density changes below the reflection level is negligible at such times (e.g., Georges, 1967). The vertical drift, V. of the reflection level is then 0.5 (AP/At) which, at and near the dip equator, represents the combined effect of electromagnetic (EXB) drift and an apparent upward drift due to chemical loss.…”

“…Earle and Kelley (1987) identified atmospheric waves and magnetospheric electric fields as the most likely sources of equatorial zonal electric field variations in the range 1-10 hr during quiet and disturbed periods respectively. Nair et al (1992) and Subbarao and Krishnamurthy (1994) suggested the frequently occurring medium scale gravity waves as the source of shorter period (<1 h) variations of V. in the evening period.…”

“…The vertical drift (V) of F region plasma (hence the causative east-west electric field) at and in the immediate vicinity of the magnetic equator is known to undergo variations on a range of time scales from about a minute to several hours during both quiet and disturbed geomagnetic conditions (e.g., Patel and La Gos, 1985;Earle and Kelley, 1987;Nair et al, 1992;Subbarao and Krishnamurthy, 1994). Earle and Kelley (1987) identified atmospheric waves and magnetospheric electric fields as the most likely sources of equatorial zonal electric field variations in the range 1-10 hr during quiet and disturbed periods respectively.…”

Inductive‐Dynamic Coupling of the Ionosphere With the Thermosphere and the Magnetosphere

“…Theoretical estimates of this time scale (discussed in section 2.4) depend on the spatial scale of the phenomenon of interest, ranging from a few seconds for purely ionospheric events to 15-30 min for changes on scales of magnetospheric dimensions. The latter is the approximate time scale for many observed important M-IT processes, such as substorms and magnetospheric transients [e.g., Earle and Kelley, 1987;Ginskey, 1993, 1995;Zesta et al, 2000;Bristow et al, 2003;Huang et al, 2008Huang et al, , 2010; the focus of our paper is on time scales from a few minutes to 20-30 min. It is, thus, not surprising that, as pointed out by Schunk [2013], conventional ionospheric quasisteady state equilibrium models, e.g., thermosphere ionosphere electrodynamics global circulation model [Richmond et al, 1992], coupled thermosphere ionosphere plasmasphere electrodynamics model [Fuller-Rowell et al, 1996], and the global ionosphere-thermosphere model (GITM) [Ridley et al, 2006], describe well the large-scale slow variations or "climatology" of the ionosphere-thermosphere system but do not work well for the rapidly changing phenomena or "weather."…”