The Ionospheric Storm

The magnetic storm is a fascinating geophysical phenomenon, which goes far beyond the visible evidence corresponding to auroral displays at high latitudes. It is central to the issues surrounding what is now referred to as space weather. A discourse on this subject is beyond the scope of this chapter, but the reader is referred to an excellent geophysical monograph edited by Tsurutani et al. [1997], Current understandings about the ionospheric storm processes appear in a paper by Buosanto [2000].

The ionospheric storm is the ionosphere's response to a geomagnetic storm. While the ionospheric responses to magnetic storms are varied, it has been shown that they may be conveniently classified as either positive or negative in nature. The main attribute of so-called negative storms is that they are generally associated with decreases in foF2. Positive storms exhibit the opposite behavior. At midlatitudes the ionospheric storm signature is typically commensurate with the main features of a negative storm, although variations may occur. Often the temporal (or stormtime) pattern is complex. For example, the midlatitude ionospheric response to a large magnetic storm is generally characterized by a short-lived increase in the F-region electron concentration in the dusk sector following storm commencement (SC), after which it decreases dramatically (see Figure 3-14). But there is also a seasonal dependence and the positive/negative phase pattern may be different for the Northern and Southern Hemispheres. The NOAA STORM model captures this difference. Referring to the Northern Hemispheric response, an initial short-lived enhancement can be observed in foF2 records and is correlated with the initial positive phase of the storm. The main phase of the geomagnetic storm is correlated with a concomitant foF2 diminution, and this reduction in foF2 may last for a day or longer. It is thought that the initial enhancement in foF2 is a result of electrodynamic forces while the long-term reduction is associated with changes in upper atmospheric chemistry and modification of thermospheric wind patterns. A key factor is atmospheric heating through dissipation of storm-induced gravity waves. This heating effect will cause the thermosphere to expand, and ionospheric loss rates will increase.

3.10.1 Early Attempts at Storm Modeling

Some of the earliest attempts to examine the impact of magnetic activity on the midlatitude ionosphere were carried out by NRL workers in the early 1970s [Goodman et al., 1971; Goodman and Lehman, 1971]. Since the work was only documented in government reports and at a meeting of the American Geophysical Union (i.e., circa 1971-72), its distribution was not broad. Nevertheless we shall dedicate a small amount of space to the main points of the NRL study in the paragraphs that follow.

The Naval Research Laboratory had an incoherent backscatter radar located at its Chesapeake Bay Division some 40 miles from Washington DC. The NRL team was able to determine a number of ionospheric parameters, including the TEC, NF2max, hF2, and the equivalent slab thickness using a hybrid Faraday rotation and Thomson scatter radar technique [Goodman, 1970]. Faraday rotation of the radar returns was used to derive an unequivocal estimate of NF2max, without resort to a non-organic sounder. Studies of correlation between previous values of the magnetic index from Fredericksburg, Virginia (i.e., K/,l{) were undertaken using midday conditions of the baseline ionosphere. The object was to derive the impulse response of the ionosphere to excursions in /T-index at different lag intervals. It is important to note that the NRL data set did not include any major storm days; the data consistent of reasonable quiet conditions. The objective was to determine whether or not there was any particular threshold effect for A^-index vis-a-vis the ionospheric impact. Having derived the correlation functions, the NRL team then exaggerated the amplitude of the ^-indices to see if they could mimic a storm. Quite surprisingly, the answer was yes. Figure 3-26

shows a typical result. The general behavior is one in which the F2 maximum height increases initially, in proportion to the amount of magnetic activity, subsequently decreases, and eventually returns to its equilibrium value. It was found that NF2max and TEC decrease with increasing lag time, with the greatest diminution occurring at ~ 24 hours on the average. On the other hand, the F2 layer scale height and the slab thickness exhibited no consistent behavior. It should be noted that these are impulse responses. A true magnetic storm response at any given time would require an integration of all of these responses, each weighted by the appropriate value of A^-index amplitude. This computed pattern of ionospheric perturbation clearly mirrors the pattern of a moderate magnetic storm, which is typically characterized by an initial positive phase followed by a longer-lasting negative main phase.

Electron Density

Figure 3-26: Response of the ionosphere to an impulse of magnetic activity. The normal unperturbed ionosphere is shown by the dashed curves, and the perturbed distributions are given by the solid curves. The scale of the perturbed distributions has been exaggerated for illustrative purposes. From Goodman [1971],

Electron Density

Figure 3-26: Response of the ionosphere to an impulse of magnetic activity. The normal unperturbed ionosphere is shown by the dashed curves, and the perturbed distributions are given by the solid curves. The scale of the perturbed distributions has been exaggerated for illustrative purposes. From Goodman [1971],

It is well known that magnetic activity is correlated with thermospheric heating. One only needs to refer to the premature loss of Skylab as a result of increased satellite drag forces to acknowledge this. Moreover, the dissipation of atmospheric gravity waves is thought to be a major source of heat in the upper atmosphere; and it is well-established that free atmospheric gravity waves and surface waves are coincident with elevated levels of magnetic activity. Since the NRL work was undertaken during quasi-quiet times, the results suggest that small, and possibly undetectable, gravity wave modes were being generated and becoming available as heat sources, even during periods of modest magnetic activity. This appears to be an important result. The reader is reminded that the NRL correlation studies correspond to data over three consecutive months in 1971 (i.e., springtime and fixed sunspot number) and the results only apply to midlatitude stations.

Workers at AFCRL (now AFRL) conducted a long-term investigation of the ionosphere, based upon TEC measurements using geosynchronous satellites, and assembled the most comprehensive data base from which magnetic storm effects could be derived [Mendillo, 1971; Mendillo, 1973; Mendillo and Klobuchar, 1974a; and Mendillo and Lynch, 1978], They developed a 62-month atlas of F-region responses to magnetic storms from which storm-time research was undertaken [Mendillo and Klobuchar, 1974b]. See Figure 3-14 for the average pattern in the variation of ionospheric parameters NF2max and TEC over 72 storms.

While most studies of the storm-time effects on the ionosphere have been directed at middle latitudes, Yeboah-Amankwah [1976] has examined eight storms and their impact at an equatorial station (i.e., Ghana). The measurements were of the TEC using the VHF Faraday rotation method, and the signal source was the geosynchronous satellite ATS-III. Yeboah-Amankwah finds a general rise in the TEC, with the largest effect at nighttime. Other equatorial investigators have also found that the correlation between the ^p-index and TEC is positive for all hours of the day. This equatorial response is different from that of the midlatitude observations.

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