Low frequency modulation of transionospheric radio wave amplitude at low-latitudes: possible role of field line oscillations

Abstract

Abstract. Ionospheric scintillations of radio waves at lowlatitudes are associated with electron density irregularities. These irregularities are field-aligned and can provide excitation energy all along the field line to non-local field-aligned oscillations, such as the local field line oscillations. Eigenperiods of toroidal field line oscillations at low-latitudes, computed by using the dipole magnetic field and ion distributions obtained from the International Reference Ionosphere (IRI) for typical nighttime conditions, fall in the range of 20-25 s. When subjected to spectral analysis, signal strength of the radio waves recorded on the 250 MHz beacon at Pondicherry (4.5 • N dip), Mumbai (13.4 • N dip) and Ujjain (18.6 • N dip) exhibit periodicities in the same range. For the single event for which simultaneous ground magnetic data were available, the geomagnetic field also oscillated at the same periodicity. The systematic presence of a significant peak in the 20-25 s range during periods of strong radio wave scintillations, and its absence otherwise suggests the possibility that field line oscillations are endogenously excited by the irregularities, and the oscillations associated with the excited field line generate the modulation characteristics of the radio waves received on the ground. The frequency of modulation is found to be much lower than the characteristic frequencies that define the main body of scintillations, and they probably correspond to scales that are much larger than the typical Fresnel scale. It is possible that the refractive mechanism associated with larger scale long-lived irregularities could be responsible for the observed phenomenon. Results of a preliminary numerical experiment that uses a sinusoidal phase irregularity in the ionosphere as a refracting media are presented. The results show that phase variations which are large enough to produce a focal plane close to the ground can reproduce features that are not inconsistent with our observations.

Examining the Applicability of Direct Analytic Method (DAM) to Normal Modes of Poloidal Oscillations Under Symmetric and Asymmetric Boundary Conditions

“…The assumption of having protons as the only ion species is not valid for low L ‐values as other species such as He + and O + etc. come into play (Sinha et al., 2002). For this reason, we restrict our computation of eigen frequencies for L > 3.0.…”

“…For low latitudes (low L ‐values), field lines are almost glued to the ionosphere and ions other than hydrogen also are to be considered either through observation or through appropriate model such as IRI‐2016 and should be appropriately fed in the analytic model. This exercise was successfully carried out for toroidal oscillations (Sinha et al., 2002) and can also be extended to poloidal oscillations. It is important to note that for idealistic ionospheric conditions, as considered in the present analysis, the mode frequency is real and hence does not account for damping.…”

Evidence of transverse magnetospheric field line oscillations as observed from Cluster and ground magnetometers

“…Cummings et al, 1969;Sinha and Rajaram, 1997) as well as at low and equatorial latitudes where heavier species such as oxygen and helium take over (e.g. Poulter et al, 1988;Sinha et al, 2002).…”

Μοντελοποίηση Διάδοσης Ραδιοκυμάτων Στην Ιονόσφαιρα Και Την Τροπόσφαιρα Με Τη Χρήση Της Μεθόδου Πεπερασμένων Στοιχείων

“…(Maruyama, 1995), (Ho et. al., 1994) και (Sinha et. al., 2002) Μία μέθοδος μοντελοποίησης του φαινομένου παρουσιάζεται από τον (Beniguel, 2002), ενώ οι επιδράσεις του σε ζεύξεις τηλεπικοινωνιών και πλοήγησης αναλύονται από τους (Basu et.…”