Hydromagnetic emissions, X-ray bursts, and electron bunches: 1. Experimental results

Abstract

Recent experimental results are presented concerning the occurrence, structure, and frequency-latitude dependence of hydromagnetic emissions (regular oscillatory micropulsations in the frequency range 0.4-7 cps). Results are also presented fo.r the occurrence of no.ise bursts (irregular micropulsations in the same frequency range). Evidence is considered suggesting that both hm emissions and noise bursts are generated by related mechanisms. The experimental results are summarized as follows: (1) Occurrence. First, short hm emission bursts sometimes occur about I min after the sudden commencement of magnetic storms. Second, simultaneous occurrence with X-ray bursts and increased riometer absorption has been noted.(2) Structure. Hydromagnetic emissions tend to occur in distinct frequency bands. The bands are usually characterized by a fine structure consisting of a superposition of repetitive wave trains of a few minutes' duration and rapidly increasing frequency. (3) Frequency-latitude dependence. An inverse relationship has been found between the highest observed hm emission frequency and the geomagnetic latitude at which the signal is observed.In this paper we consider micropulsations of still shorter periods, that is, signals in the frequency range near I cps. More specifically, a band pass of 0.4-7 cps is employed. Most micropulsations: of the types of interest to us here occur in this general frequency range. However, the actual lower and upper limits of the pass band, 0.4 and 7 cps respectively, were determined by the design parameters of the detection equipment [Tepley, 1961b].The micropulsations are placed in two general categories. SignMs in one of the categories, referred to below as 'noise bursts,' are closely related to micropulsations of longer periods. Sig-nMs in the other category, which are the primary concern of this paper, are les. s clo.sely related to other types of geomagnetic fiuctuatio.ns. We refer to micropulsations in the latter cate-3317 3318 TEPLEY AND gory as 'hydromagnetic emis'sions,' since it has been demonstrated in separate studies that the properties of the signals are consistent with their generation either above or high in the ionosphere and their subsequent propagation downward through the ionosphere by hydromagnetic waves [Wentworth, 1961; Tepley, 1962] (also private communications from H. Benioff and G. Bodvarsson concerning unpublished work at California Institute of 'Technology). The term 'emission' was introduced to suggest an analogy between these signals and the so-called 'VLF emissions,' which are generated above the ionosphere [Gallet and Helliwell, 1959; Gallet, 1959]. The terms 'hm emission' and 'noise burst' are defined below. In the appendix, they are again considered in relation to nomenclature (pearls, SIP, IPDP, type A and Wpe B oscillations, and solar whistles) currently employed by other workers to describe signals in the same general frequency range.

Recent Investigations of Hydromagnetic Emissions Part II. Theoretical Interpretation

“…The rising-frequency fine structure is attributed to a latitude variation of the properties of the lower exosphere, which give rise to the hydromagnetic resonance effect. However, a serious drawback is that the model predicts that the emission frequency should increase with geomagnetic latitude, a prediction in conflict with experimental observations (Tepley and Wentworth, 1962b;Heacock and Hessler, 1962;Gendrin, 1963b, his Figure 1;Matveeva and Troitskaya, 1963;Wentworth, 1964). Since this model also seems unlikely to be responsible for the generation of hm emissions, it will no longer be considered in the present paper.…”

“…experimental reviews have been written by Jacobs and Watanabe, 1963a;Campbell, 1964;, 1966. In particular, hm emissions consisting of a band of frequencies (typically d f/f=N0.5 where d f is the bandwidth and f is the center (257) can usually be interpreted as a series of repetitive overlapping wave trains of rapidly rising frequency (typically d f/dt varies between 0.1 and 0.5 cps/min; Tepley and Wentworth, 1962b). Examples of such spectra are shown in Part I, Figures 2 and 3.…”

Geomagnetic disturbances on ground associated with particle precipitation during SC

“…In accordance with Tepley and Wentworth (1962) and Safargaleev et al (2002), approximately 20% of SWPPs are accompanied with excitation of a narrowband burst (or series of bursts) of rising frequency like that shown in Fig. 2b and Fig.…”

“…5). Such kind of ULF activity, firstly described by Tepley and Wentworth (1962), seems to be the most typical geomagnetic response to the SWPP in the frequency range 0.1-3 Hz (e.g. Kangas et al, 1998).…”

“…It is well established that the solar wind pressure pulse is associated with a step-like increase of the geomagnetic H-component at low latitudes (e.g. Araki, 1994), pulsations of Pc 5 and Pc 1 frequency ranges at higher latitudes (e.g., Saito and Matsushita, 1967;Tepley and Wentworth, 1962), the enhancement of ionospheric absorption (e.g., Ranta and Ranta, 1990) and the intensification of electron and proton auroras (e.g., Brittnacher et al, 2000;Hubert et al, 2003). These effects and inter-relations between them are studied in the present paper.…”