Howard F. Bates

The southward midnight surge in F-layer wind observed with the Chatanika incoherent scatter radar

Abstract Long duration incoherent-scatter data from the Chatanika radar have been analyzed to obtain the meridional component of the neutral wind in the sunlit F -layer thermosphere. In the cases examined, the meridional wind exhibits a southward surge of 150 to 500 m/s around local midnight. Joule heating does not account for the surge because the surge occurs close to local midnight in each of the cases examined, whereas the onset time of appreciable local heating varies from 3 to 6 h earlier. The magnitude of the surge correlates highly with planetary magnetic activity. It has been proposed by others that the ionospheric electric field sets up two great counter-rotating convection vortices in the polar cap thermosphere at altitudes near the sunlit F -layer maximum. The surge is concluded to be the result of the passage over the radar of the narrow region of southward wind dividing the vortices. The midnight surge may be a daily source of thermospheric waves as the Earth rotates beneath it.

On the correlation of optical and radio auroras

Abstract A comparison of simultaneous optical and radio auroral data obtained during 12 days in December, 1964, shows conclusively that the radio wave scattering belt includes the visual auroral belt. The optical data were obtained with the Alaskan network of all-sky cameras located between 65 and 80° north geomagnetic latitude. The multifrequency backscatter sounder operating at College was used to record the radio data. The excellent agreement that resulted leads us to conclude that the majority of the h.f. and low v.h.f. backscatter echoes originating in the E - and the F -regions at high latitudes are auroral echoes in the strict sense of the term. In the records shown the auroral belt was tracked from 65 to above 80° north geomagnetic latitude by a single sounder. It is concluded that the multifrequency h.f. backscatter sounder makes year-round observations of the auroral belt possible.

Atmospheric expansion through Joule heating by horizontal electric fields

Abstract Incoherent scatter measurements made along a magnetic field line into aurora during a period of high electric field in the recovery phase of a substorm show (1) considerably increased electron densities well above the normal F-region maximum, and (2) field-aligned plasma drifts that increase with altitude. A model invoking atmospheric expansion through Joule heating by the horizontal electric field driving the auroral electrojet is used to explain the observations. From this study it is concluded that during magnetically disturbed periods (1) Joule heating by the auroral electrojet raises the neutral temperature and density in the auroral zone ionosphere at F-region heights, (2) ionization formed by the aurora is transported upward by the expanding atmosphere, at times producing an appreciable increase in lower exospheric plasma densities on the field lines containing the aurora, and (3) combined satellite, radar, and optical observations during periods of aurora and high electric field could provide measured F-region collision frequencies.

Thermospheric changes shortly after the onset of daytime joule heating

In the first few tens of minutes after the onset of widespread Joule heating, the motion of the ionospheric atmosphere can be approximated by the one-dimensional motion of a gas in a gravity field—a problem that is easily solved because the motion takes place at constant pressure. The solution provides an estimate of time for which the model is applicable to the physical situation. Seasonal variations of the early effects are examined by using ion profiles appropriate to each season. The results show that the atmosphere above 100 km is strongly modified within a few tens of minutes after the onset of widespread heating: the density can double, the temperature can increase several hundred degrees, and the molecular nitrogen concentration can quadruple. Vertical winds exceeding 100 m/sec at 400km altitude are possible for a brief period after the onset of electric fields of 100 mV/m—rare but observed events. In the first few tens of minutes after the onset of a given electric field, the greatest power is deposited in the thermosphere around summer solstice, while the greatest winds occur at 200 km altitude in the summer and at 400km in the winter. These differing seasonal effects show primarily that a given level of change occurs sooner for one season than another, not that long term seasonal differences exist. Once a magnetic storm is in progress, the quiet-day ion profiles change to the non-seasonal storm profile ; for this ion distribution, F-region effects are minimum regardless of season. Joule heating effects in the upper thermosphere are therefore concluded to be self-limiting.

Atmospheric expansion from Joule heating

Abstract An expression for the vertical velocity of the neutral atmosphere in the F-region is derived for Joule heating by the electric field that drives the auroral electrojet. When only vertical expansion is allowed, it is found that the vertical wind must always increase monotonically with altitude. The heating rate is proportional to the F-region ion density, so that appreciable heating, even during high electric fields, requires some production mechanism of ionization such as auroral secondary ionization or solar photoionization, in the lower F-region. Once started at night, when an ionizing source is present in the lower F-region, the expansion of the atmosphere transports ionization upward, thereby increasing the heating rate, and hence the expansion rate, i.e. positive feedback. Electric field strengths and F-region ion densities of 50 mV/m and 2 × 1011e/m3, respectively, will produce vertal neutral wind speeds of several tens of m/sec in the 300–500 km altitude range. During periods of high magnetic activity, i.e. high electric field, Joule heating can produce large increases in the relative N2 concentration in the upper F-region; computations made with a simple model suggest that tenfold increases can occur at 400 km altitude 1 2−1 hr after the onset of magnetic activity, a result in agreement with satellite observations. When the Joule heating theory is applied to incoherent scatter data taken during one period of high heating, the horizontal electric field in the F-region is found to decrease markedly, possibly approaching zero as the field penetrates a weak, discrete auroral arc; the decrease began 10–20 km from the arc.

VLF observations at college, Alaska, of various D-region disturbance phenomena

Abstract Between late 1961 and mid-1964 several stabilized VLF transmitters have been monitored at College, Alaska. During that period 1846 optically detected solar flares were observed when our NBA recorder was operating and the path was completely sunlit. Of these, 66 produced phase anomalies on our NBA records. The probability of occurrence increased approximately linearly with flare area, but was less than probabilities for SIDs determined by other methods. This indicates the VLF phase is not as sensitive as other SID indicators. The SPA decay was very nearly exponential on the College signals. The decay was significantly faster at 10.2 kc/s than at 20 kc/s frequencies. Polar cap precipitation events disrupt polar VLF propagation. The polar D -region was depressed significantly for periods of up to four or five days after relatively minor PCA events. Immediately after three events the phase from GBR was advanced 180 to 270 degrees beyond normal daytime values. VLF phase anomalies produced by (1) the U.S. and U.S.S.R. high altitude nuclear explosions in 1962, (2) the 20 July 1963 solar eclipse, and (3) the 1 October 1961, sub-auroral-latitude particle precipitation event are shown and discussed.

A proposed polar auroral radar system

Abstract The position of the auroral belt can be reliably determined through the use of an h.f. multifrequency backscatter sounder. Auroral echoes recorded at Thule show that aspect-sensitive auroral backscatter can be obtained from the south in the northern hemisphere. In comparing simultaneous Thule and College records, the auroral belt latitude data were found to be independent of the latitude of the auroral radar. It is proposed that one multifrequency h.f. backscatter sounder installed at the north geomagnetic pole could provide real-time information on the position of the entire auroral belt around the northern hemisphere.

A technique for using incoherent scatter to estimate F-region zonal winds during Joule heating

Abstract The zonal neutral wind speed in the F-region is difficult to measure when the thermosphere is sunlit; this paper presents a simple, readily applied technique employing incoherent scatter measurements during Joule heating events. At the F-layer peak (where neutral and ionic masses are equal), equal amounts of energy are imparted by the ionospheric electric field during ion-neutral collisions to the ion and neutral gases; because there are far fewer ions than neutrals, the ion temperature rapidly rises, thereby increasing the rate that heat is transferred from ion to neutral gases through ion-neutral collisions. The key to the technique is this result: At the sunlit F-layer peak, this heat transfer (which depends linearly upon the ion-neutral temperature difference) is so rapid that it approximately equals one-half the input Joule power, which in turn depends directly upon the square of the relative Hall ion drift speed. The ion temperature is measured, but during Joule heating the neutral temperature cannot be found from incoherent scatter measurements; however, by finding limits for the neutral temperature, limits on the ion-neutral heat transfer are readily found. Because the total input Joule power is twice the increase in ion-neutral heat transfer, these limiting values provide limits on the relative Hall speed. The zonal wind is the dominant wind component in the relative Hall speed (and the meridional wind speed is measurable), so the method yields upper and lower limits for the zonal wind during Joule heating. The upper neutral temperature limit is derived by assuming that the thermosphere expands only vertically; to ensure that an upper limit is obtained, all loss terms except adiabatic expansion at constant pressure are dropped. The lower neutral temperature limit is obtained by assuming that the Joule heating does not increase the initial neutral temperature. In the two cases studied to date these nearly opposite assumptions about the energy balance of the thermosphere lead to zonal wind limits that are not greatly different and that thereby provide useful estimates. It is concluded from these events that the neutral wind at the sunlit F-layer peak responds so rapidly to ion drag that the effective electric field averages no more than one-half the applied magnetospheric electric field during the time when E > 15 mV/m and is increasing. This result suggests that the ion drift and neutral wind velocities in the sunlit polar cap are nearly parallel and of comparable magnitude.

Computed bounds on the F-region zonal wind during Joule heating

Abstract A method was derived in previous work that allows the computation from incoherent scatter data of upper and lower bounds on the thermospheric temperature and zonal wind speed during Joule heating. The theory is derived here more rigorously, and an approximation is made to include the effects of vertical heat conduction in the upper bound. For illustrative purposes, the resulting bounds are computed using two sets of experimental data. Including conduction markedly decreases the upper bound in neutral temperature from that found without conduction. In the examples shown the average difference between zonal wind upper and lower bounds with conduction is found to be 12%, whereas without conduction it is over 40%. Because of the simplifying approximations made, the technique provides an estimate of the zonal wind only under specialized conditions. However, the results are useful because they can be interpreted to apply generally to the motion of the sunlit F-region thermosphere.