Natural sources of geomagnetic field variations

The Earth’s magnetic field is a dynamic system and varies on a wide spectrum of timescales from microseconds to hundreds of millions of years. The primary source of the field is the self-sustaining geodynamo action of the Earth’s liquid outer core. This creates around 95% of the magnetic field strength at the Earth’s surface. Its average strength at mid-latitudes is on the order of 50,000 nT (ranging between 20,000-60,000 nT increasing toward the poles). The core field varies on timescales of years to millennia. Another internal source is the quasi-stable crustal field, generated by the heterogeneous distribution of ferromagnetic minerals in the upper 5-30 km of the Earth’s surface. Its contribution is much smaller at around 20 nT on average globally, though it can locally be much larger. It changes on timescales of millions of years except at sources such as active volcanic regions or along mid-ocean ridges. There are a number of external (i.e. with sources outside the Earth) field systems which are created by solar-terrestrial interactions. These are much more dynamic and vary on timescales of seconds to days. These have magnitudes of a few pT to 100 nT on geomagnetically quiet days but can change rapidly within minutes to thousands of nT, for example from the impact of an Interplanetary Coronal Mass Ejection upon the Earth. These effects (geomagnetic storms and substorms) are strongly dependent on local time and latitude, with high latitudes (|Φgeomagnetic| > 60°) being particularly affected from the auroral electrojet current systems or magnetospheric waves. Due to simple geometric reasons (zonal currents), most of the above geomagnetic disturbances appear in the geomagnetic north (also called the horizontal) component. Other magnetic fields are generated locally by instantaneous phenomena such as lightning-generated spherics and magnetospheric whistlers. We will briefly outline the spatio-temporal variation and largest dynamic expected from each source. In this concise review we focus on mid-latitudes (CERN is located at 46.2° geographic latitude, 40.4° geomagnetic latitude, at the footpoint of the L=1.8 magnetic McIlwain-shell) and neglect some of the high-latitude/auroral and equatorial phenomena not relevant for CERN’s location

Remote sensing of D-region ionosphere using multimode tweeks

Lightning discharges radiate electromagnetic waves in a wide frequency range, with maximum energy in extremely low frequency/very low frequency band. A part of the radiated extremely low frequency/very low frequency wave energy is trapped in the Earth–ionosphere waveguide and travels thousands of kilometers in different modes with lower attenuation. Amplitude, frequency and phase of these waves are used to study the less explored D-region ionosphere at lower latitudes. Extremely low frequency/very low frequency observations are recorded continuously by automatic whistler detector setup installed at low-latitude Indian station Lucknow (Geom. lat. 17.6 ° N; long. 154.5 ° E). In total, 149 cases of tweeks having modes ranging from 3 to 6 have been recorded by automatic whistler detector during December 2010 and analyzed. Result shows that the propagation distance in the Earth–ionosphere waveguide lies between 1.1 and 9.4 Mm. The electron density in the lower D-region varies between 25 and 150 cm - 3 . The upper boundary of the waveguide varies between 80 and 95 km. The reported results are in good agreement with the earlier measurements at different latitudes and longitudes

Very low latitude (L = 1.08) whistlers and correlation with lightning activity

We present analysis of more than 2000 lightning-generated whistlers recorded at a low-latitude station, located at Allahabad (geographic latitude, 25.40N; geographic longitude, 81.93E; L = 1.081), India, during December 2010 to November 2011. The main focus of this work is on the correlation between observed low-latitude whistlers and lightning activity detected by the World-Wide Lightning Location Network near the conjugate point (geography 9.87S, 83.59E) of Allahabad. Whistler occurrence is higher in the postmidnight period as compared to the premidnight period. Whistlers were observed in the daytime only on 2 days that too before 8:30 LT (morning). Seasonally, occurrence is maximum during winter months, which is due to more lightning activity in the conjugate region and favorable ionospheric conditions. About 63% of whistlers were correlated with lightning strokes in the vicinity of the conjugate point within spatial extent of 1000 km (conjugate area). Most (about 53%) whistlers were found to be associated with lightning strokes that were offset to the southeast of the conjugate point. The results indicate that an energy range of 7.5-17.5 kJ of lightning strokes generate most of whistlers at this station. The L shell calculations show that propagation paths of the observed whistlers were embedded in the topside ionosphere. Based on these results we suggest a possibility of ducted mode of propagation even for such very low latitude whistlers. ©2015. American Geophysical Union. All Rights Reserved