James A. Whalen

AIRBORNE IONOSPHERIC AND OPTICAL MEASUREMENTS OF NOONTIME AURORA.

Abstract The results are reported of a series of three local noon flights by the AFCRL Flying Ionospheric Laboratory through the latitude of the auroral oval. The flights, which were performed in December 1968 over the Arctic Ocean, maintained local magnetic noon while scanning back and forth in latitude for durations of 5–10 hr each for a total of 16 latitude scans and 20 hr all in darkness and at nearly constant K p (2- to 3-). Coordinated in these airborne measurements were an ionospheric sounder, all sky cameras and photometers. Parameters analyzed are f 0 E, h′E, h′E s , f min ; the presence of discrete aurora overhead; the intensities of N 2 + 4278 A, OI 5577 A and OI 6300 A measured by a photometric technique which discriminates between discrete and non-discrete aurora. An auroral form which is spatially continuous and slowly varying in time is detected simultaneously by the ionospheric sounder as an E -layer and by the photometer as a non-discrete component of N 2 + , the intensity varying as the fourth power of E -layer critical frequency. This continuous aurora: 1. (1) extends in an unbroken band typically 5° equatorward from the instantaneous position of the discrete auroral oval which falls between 75° and 78° corrected geomagnetic latitude; 2. (2) produces an E -layer the virtual height of which corresponds to precipitating electrons whose particle energies decrease from 3 keV at the low latitude edge to 1 keV at the high latitude edge where the E -layer merges with a band of enhanced non-discrete 6300 A emission at the same location as the discrete aurora; 3. (3) terminates at high latitude under the discrete aurora where the E -layer transforms to sporadic E at the same virtual height; 4. (4) has maximum precipitation energy flux of 0.08 ergs/cm 2 sec which decreases with increasing latitude; 5. (5) has a particle flux of 2 × 10 7 electrons/cm 2 sec; 6. (6) frequently overlaps near the low latitude edge with a zone of D -region ionization which extends equatorward at least 7° from 74°; and 7. (7) together with the discrete aurora and non-discrete 6300 A band constitutes the ‘soft’ zone of electron precipitation measured by satellite, the D -layer zone corresponding both to the satellite ‘hard’ zone and to the auroral absorption zone.

The equatorial anomaly: Its quantitative relation to equatorial bubbles, bottomside spread F, and E×B drift velocity during a month at solar maximum

The scintillation that is most disruptive to trans-ionospheric RF propagation occurs when an equatorial bubble intersects the maximum electron density of the equatorial anomaly. This paper reports the first systematic observational study of the important interrelation between bubble and the anomaly together with the maximum pre-reversal E × B drift velocity, and strong bottomside spread F (BSSF), which is a necessary condition for bubbles. An array of ionospheric sounders located near 75° W longitude measures latitudinal profiles of NmF2 at hourly intervals through 0°–40° dip latitude (DLAT) and 1800–0300 LT during a continuous period of 30 days at equinox and solar maximum. The anomaly is highly variable from day to day, but at 2100 LT, the time of its highest latitude, crest latitude and magnitude increase and decrease together, a relation that is linear above a threshold with coordinates of 38 × 105 el/cm3 and 15.4° DLAT. This threshold is important also because it corresponds to the maximum drift velocity of 50 m/s, and because above it nearly all bubbles are observed, directly as macroscopic bubbles and indirectly as strong BSSF. The importance to C/NOFS and other satellites is that maximum E × B drift, crest NmF2 and crest DLAT are in apparent one-to-one correspondence above this threshold, so that measurement of any one of the three implies measurement of the other two. Furthermore, any such measurement that exceeds the threshold indicates that bubbles can occur, whereas one that falls below the threshold indicates that they cannot.

Appleton anomaly increase following sunset: Its observed relation to equatorial F layer E × B drift velocity

The upward E × B drift velocity of the postsunset F layer at the dip equator, in response to the prereversal electric field, and the resulting enhancement of the fountain effect plasma at the Appleton anomaly are observed nearly simultaneously near the same geomagnetic meridian. Measurements, the first under such controlled conditions, are by a chain of ionospheric sounders recording ionograms at intervals of 5–15 min on two consecutive days during equinox at solar maximum, both having quiet magnetic conditions and high solar fluxes. Measured is the following sequence of cause and effect: the equatorial F layer electron density that persists through sunset; the solar E layer electron density conjugate to this F layer that decays rapidly through sunset; the bottomside equatorial F layer altitude, h′F, as it drifts upward in response to the F region dynamo electric field; and the anomaly F layer electron density that increases with the arrival of the resulting enhanced fountain effect plasma at 16.0° and at 20.3° dip latitude. At the equator the maximum drift velocity measured by dh′F/dt is 70 m s−1 on the first day and 50 m s−1 on the second. The differences in drift velocity are reflected in the anomaly in latitudinal distributions of F layer maximum electron density between 0° and 30° dip latitude that show the anomaly crest to increase in magnitude and in latitude: rapidly on the first day, reaching a maximum of 55 × 105 el cm−3 18° dip latitude at 2100 LT, and more slowly on the second day, reaching ≥38 × 105 el cm−3 at ≤16° dip latitude, also at 2100 LT. In addition to the qualitative relationship, maximum drift velocity and anomaly crest maximum electron density and its maximum rate of increase are all reduced by about the same factor on the second day relative to the first.