Estimating the temperature and density of a spicule from 100 GHz data obtained with ALMA

TL;DR

This study observed and analyzed a solar spicule using ALMA, IRIS, and AIA, estimating its physical parameters and classifying it as a macrospicule without a hot sheath.

Contribution

First simultaneous multi-instrument observations of a solar spicule at 100 GHz and other wavelengths, with physical parameter estimation and classification.

Findings

Estimated temperature ~6800 K and density 2.2 x 10^10 cm^-3 for the 100GHz spicule.

The 100GHz spicule is likely a macrospicule without a hot sheath.

The 100GHz spicule is physically distinct from the MgII/304A spicule.

Abstract

We succeeded in observing two large spicules simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. One is a spicule seen in the IRIS Mg II slit-jaw images and AIA 304\AA\ images (MgII/304A spicule). The other one is a spicule seen in the 100GHz images obtained with ALMA (100GHz spicule). Although the 100GHz spicule overlapped with the MgII/304A spicule in the early phase, it did not show any corresponding structures in the IRIS Mg II and AIA 304A images after the early phase. It suggests that the spicules are individual events and do not have a physical relationship. To obtain the physical parameters of the 100GHz spicule, we estimate the optical depths as a function of temperature and density using two different methods. One is using the observed brightness temperature by assuming a filling factor, and the other is using an emission model for the optical depth. As a result of comparing them, the kinetic temperature of the plasma and the number density of ionized hydrogens in the 100GHz spicule are ~6800 K and 2.2 x 10^10 cm^-3. The estimated values can explain the absorbing structure in the 193A image, which appear as a counterpart of the 100GHz spicule. These results suggest that the 100GHz spicule presented in this paper is classified to a macrospicule without a hot sheath in former terminology.