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ABSTRACT We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival Green Bank Telescope (GBT), Murchison Widefield Array, and Very Large Array (VLA) images at 333, 88, and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud associated with the halo of the Sgr B complex on a scale of ∼60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic ray electrons. Correlation of the Fe i 6.4 keV K α line and synchrotron emission provides compelling evidence that the low energy cosmic ray electrons are responsible for producing the K α line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has a mean spectral index α ∼ −1 ± 1, which gives the magnetic field strength ∼100 µG for cloud densities nH = 104–105 cm−3, and estimated cosmic ray ionization rates between 10−13 and 10−14 s−1. Furthermore, the energy spectrum of primary cosmic ray electrons is constrained to be E−3 ± 1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and γ-ray emission detected from this cloud. These measurements have important implications on the role that high cosmic ray electron fluxes at the Galactic centre play in production of radio synchrotron emission, the Fe i K α line emission at 6.4 keV, and ∼GeV γ-ray emission throughout the Central Molecular Zone. 

Abstract This paper describes a new method for remote sensing of magnetic field fluctuations at ionospheric altitudes using a relatively long‐baseline interferometer and exceptionally bright cosmic radio sources at 35 MHz. The technique uses sensitive measurements of the difference in phase between two phased array telescopes separated by about 75 km and between the right and left circular polarizations to measure the amount of differential Faraday rotation. Combined with estimates of the background magnetic field and total electron content, these can be converted to measurements of fluctuations in the differential magnetic field parallel to the line of sight, ΔB_‖. The temporal gradient in ΔB_‖roughly follows the diurnal pattern expected for B_‖due to the vertical gradient in the background electric field, but at roughly 25% the magnitude and offset by ∼50 nT hr⁻¹. This suggests that the diurnal variation in the electric fields observed by the two telescopes are similar but slightly different (|ΔE| ≲ 0.1 mV m⁻¹). Fluctuations in ΔB_‖were typically ∼10–30 nT with wavelike fluctuations often apparent. These typically have oscillation periods of about 10–30 min, similar to traveling ionospheric disturbances (TIDs). Simultaneous observations toward two sources separated by 25.4° on the sky (∼140 km in the F‐region) show a few detections of wavelike disturbances with lags of ±10–30 min between them. These imply speeds on the order of 100–200 m s⁻¹, also similar to TIDs. We estimate that gravity waves with amplitudes within the dynamo region of ∼10 m s⁻¹could generate the observed fluctuations in ΔB_‖.