Daytime midlatitudeDregion parameters at solar minimum from short-path VLF phase and amplitude

Abstract

[1] Observed phases and amplitudes of VLF radio signals propagating on a short (∼360 km) path are used to find improved parameters for the lowest edge of the (D region of the) Earth's ionosphere at a geomagnetic latitude of ∼53.5°in midsummer near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured both near (∼110 km from) the transmitter, where the direct ground wave is very dominant, and at distances of ∼360 km near where the ionospherically reflected waves form a (modal) minimum with the (direct) ground wave. The signals came from the 24.0 kHz transmitter, NAA, on the coast of Maine near the U.S.-Canada border, propagating ∼360 km E-NE, mainly over the sea, to Saint John and Prince Edward Island. The bottom edge of the midday, midsummer, ionosphere at ∼53.5°geomagnetic latitude was thus found to be well modeled by H′ = 71.8 ± 0.6 km and b = 0.335 ± 0.025 km −1 where H′ and b are Wait's traditional height and sharpness parameters used by the U.S. Navy in their Earth-ionosphere VLF radio waveguide programs. The variation of b with latitude is also estimated with the aid of interpolation using measured galactic cosmic ray fluxes.

Low‐latitude ionospheric D region dependence on solar zenith angle

“…However, as for the NWC‐Kyoto path, H′ and β at midday could be estimated fairly well from previous measurements at low and middle latitudes near solar maximum, taking into account known variations of midday H′ and β with latitude, in particular, for NAU‐St. John's, decreases in β with increasing magnetic latitude due to the corresponding galactic cosmic ray intensity increases [ Thomson et al, , , ].…”

DaytimeDregion parameters from long-path VLF phase and amplitude

“…These average observed values of H ′ = 71.1 km and β = 0.42 km −1 for the long NWC‐Tumwater path can usefully be compared with the values H ′ = 70.5 km and β = 0.47 km −1 for the short (300 km) low‐latitude (∼30° geomagnetic) NWC‐Karratha path (for near‐overhead Sun) [ Thomson , 2010] and the values H ′ = 71.8 km and β = 0.34 km −1 for the short (360 km) high‐midlatitude (∼53.5° geomagnetic) NAA‐PEI path (for near‐overhead Sun) [ Thomson et al , 2011]. The latter paper also gives a graph of β versus geomagnetic latitude interpolated using the known latitudinal variation of galactic cosmic ray fluxes.…”

“…For the short (∼300 km) low‐latitude path, from NWC to Karratha, on the coast of NW Australia (∼20°S geographic, ∼30°S geomagnetic; see Figure 1), Thomson [2010] used VLF observations plus ModeFinder to determine H ′ = 70.5 km and β = 0.47 km −1 near midday in late October 2009 (i.e., with the Sun near the zenith). Similarly, for the short (∼360 km) high‐midlatitude path, NAA (Maine) to Prince Edward Island, Canada (∼46°N geographic, ∼53.5°N geomagnetic), Thomson et al [2011] used VLF observations plus ModeFinder to determine H ′ = 71.8 km and β = 0.34 km −1 near midday in June and July 2010 (i.e., with the Sun again near the zenith). The lower β at the higher‐latitude site was attributed to the much higher galactic cosmic ray fluxes at higher latitudes and enabled a tentative plot of β versus geomagnetic latitude to be produced.…”

“…LWPC and ModeFinder generally give very similar results but, because LWPC is set up to automatically take into account changes in the geomagnetic dip and azimuth along the path, it is used for the long paths here. Changes in H ′ and β that are due to changing solar zenith angles along the path can be found from the works of Thomson [1993] and McRae and Thomson [2000], while changes in β that are due to changing geomagnetic latitudes can now also be allowed for from the plot of Thomson et al [2011], mentioned above. Changes in H ′ with latitude and season depend effectively on the height changes of a fixed neutral density near 70 km altitude and can be estimated from the Mass Spectrometer Incoherent Scatter‐ (MSIS‐) E‐90 neutral atmospheric density model [http://omniweb.gsfc.nasa.gov/vitmo/msis_vitmo.html].…”

Upper atmospheric remote sensing using ELF – VLF lightning generated tweek and whistler sferics

“…Nighttime D-region ionization is mainly caused by Lyman-α and Lyman-β which ionize NO and O 2 . The Galactic Cosmic Rays of which intensity is the highest during solar minimum and the lowest during solar maximum are another important source of ionization of the nighttime D-region below heights of 65 -70 km, while above these heights, the Lyman- radiation dominates (Thomson et al, 2011). However, the main source of nighttime D-region variability appears to be atmospheric gravity waves (AWGs) of different origins including meteorological origin such as strong thunderstorms that can generate the acoustic and gravity waves (Lay et al, 2015).…”