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

Whalen, J. A.

doi:10.1029/98ja00104

Cited by 11 publications

(12 citation statements)

References 19 publications

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“…The anomaly maximum latitude at 2100 LT has been found here to be related to the maximum eastward electric field as represented by the prereversal maximum E x B drift velocity which occurs near 1830 LT, nearly 2.5 hours earlier [Whalen, 1998]. Furthermore, drift reversal intervenes at -•1920 LT, so that for nearly 1.5 hours prior to the 2100 LT maximum, the anomaly plasma is under the control of the westward electric field.…”

Section: Postreversal E X B Driftmentioning

confidence: 90%

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

Whalen¹

2001

J. Geophys. Res.

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Abstract. 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 x B drift velocity, and strong bottomside spread F (BSSF), which is a necessary condition for bubbles. An array of ionospheric sounders located near 75 ø W longitude measures latitudinal profiles of NmF2 at hourly intervals through 0o-40 ø 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 x 105 el/cm 3 and 15.4 ø 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 x 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.

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Section: Postreversal E X B Driftmentioning

confidence: 90%

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

Whalen¹

2001

J. Geophys. Res.

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“…Following onset, oblique RSF echoes from both the east and west boundaries located the bubble continuously and redundantly at both equator and anomaly and measured its eastward velocity at both locations. A second paper measured the dynamic relation between E x B drift of the bottomside F layer at the dip equator and the resulting increase of the NmF 2 at the Appleton anomaly nearly simultaneously near the same geomagnetic meridian [Whalen, 1998]. This first direct observation indicated that on the two days studied, the maximum rate of increase in NmF2 and its maximum magnitude were quantitatively related to the maximum E x B drift velocity.…”

Section: Observational Approach and Outlinementioning

confidence: 97%

“…To illustrate the how the change of a single parameter can change this result, if the LT of the anomaly maximum had occurred at 2300 LT (its value observed on September 13, 1958 [Whalen, 1998] Additional comprehensive observations are possible using the IGY sounder array because it operated on this same scale routinely on every day throughout the year thus affording the opportunity to examine a large range of conditions. Since there were at least 99 macroscopic bubbles evident in the hourly tabulations of foF2 at anomaly locations throughout the year [Whalen, 1997], the prospect is good for finding cases in which the effects can be identified and the causes investigated retrospectively.…”

Section: Variability Of Bubble-anomaly Intersectionmentioning

confidence: 99%

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An equatorial bubble: Its evolution observed in relation to bottomside spread F and to the Appleton anomaly

Whalen¹

2000

J. Geophys. Res.

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Abstract. This is a case study that undertakes the first comprehensive description of the concurrent evolutions of an isolated equatorial bubble, bottomside spread F (BSSF), and the enhanced Appleton anomaly. A principal focus is the dynamics of the intersection of the bubble with the anomaly crest which is the source of maximum scintillation on transionospheric RF propagation. Conditions are equinox at solar maximum. Measurements were by an array of eight ionospheric sounders in the Western Hemisphere with a collective field of view extending along the dip equator for about 1600 km and to 40 ø north dip latitude that resolved rapid changes in the phenomena yet encompassed their extent and duration. Viewed locally, the bubble was triggered by an upwelling of the bottomside F layer during E x B upward drift that accelerated upward for 15-20 min before the onset of irregularities seen as range spread F (RSF). Viewed macroscopically, this onset was part of continuous front of RSF onset that coincided with E x B reversal and moved westward at the velocity of the terminator so as to remain at 1920 LT, to the east of the bubble as BSSF; within the bubble during the upwelling of the bottomside crest; at the west wall of the bubble at the onset of its eastward drift; and west of the bubble as BSSF. The bubble drifted eastward at 185 m/s and rose upward at -<190 m/s intersecting the maximum of the anomaly crest at 2100 LT, 95 min after bubble onset. During this entire period the bubble was in continuous contact with the anomaly profile which was simultaneously increasing but as a function of LT. Thus the intersection was a function of the upward and eastward velocities of the bubble and the westward motion of the anomaly at the velocity of the terminator. The time and place of the intersection is thus dependent on the entire sequence of events and on the differing dependences on UT and LT, and since each can vary, the time and place of maximum scintillation is highly variable. In light of this variability, observations at least as comprehensive as those reported here appear to be necessary to describe the region adequately and therefore to study it further.

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“…The pre-midnight brightening that we see so prominently in the winter months is due to the enhanced fountain effect described by Whalen (1998). He studied the EIA using IGY data from a chain of South American stations to show that the following sequence occurs.…”

Section: Intertropical Arc Morphologymentioning

confidence: 99%

Brightness variations of the northern 630nm intertropical arc and the midnight pressure bulge over Eritrea

et al. 2004

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Abstract.The nightglow brightness at 630 nm from the thermospheric O( 1 D) layer was monitored nightly at Asmara, Eritrea (15.4 • N, 39.9 • E, 7 • N dip) with an all-sky imager. Averages of north-south strips of the images enabled contour plots of brightness on a latitude vs. local time grid. The contours show the movement of the intertropical arc southward before midnight, staying just north of Asmara after midnight, and gradually brightening to a maximum at 02:00 h local civil time, 02:00 LT, after which it disappears before dawn. It is argued that all features of the plots can be explained by known mechanisms capable of driving ions along magnetic field lines, including the fountain effect, summer to winter transequatorial winds, and the midnight pressure bulge.The 02:00 LT brightness maximum is the most striking and the most persistent feature in the data. The persistence of the location of the 02:00 LT brightening is attributed to a pressure bulge centered on the geographic equator at midnight and extending to higher latitudes with increasing local time in both the winter and the summer hemispheres. The bulge is shown to be stronger near solstice than near equinox, confirming earlier work.

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

Cited by 11 publications

References 19 publications

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

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

An equatorial bubble: Its evolution observed in relation to bottomside spread F and to the Appleton anomaly

Brightness variations of the northern 630nm intertropical arc and the midnight pressure bulge over Eritrea

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