Photo voltaic spicules have been found by the Jesuit priest Angelo Secchi in 1877. They’re small plasma jets of some hundred kilometers in diameter which are launched to heights >10 Mm with speeds starting from a number of 10s of km/s to >100 km/s. Individually they’ve lifetimes of some minutes. They happen within the chromospheric community over the whole lot of the Solar. Early estimates (Beckers 1972) steered that spicules carry an upward mass flux 100 occasions that required to replenish losses from the corona as a result of photo voltaic wind. With the arrival of recent, high-resolution observations from house utilizing Hinode SOT (optical), IRIS (UV), and SDO AIA (EUV), coupled with superior modeling, there was a resurgence of curiosity in spicules and their doable function within the mass and vitality price range of the photo voltaic ambiance.
Observations at millimeter wavelengths (m-$lambda$) are extremely complementary to these within the O/UV/EUV bands. It is because the spicule emission is free-free from thermal electrons in LTE, the supply operate at mm-$lambda$ is Planckian and the Rayleigh-Denims approximation is legitimate. In distinction, O/UV strains type in non-LTE. Observations of the Solar generally and spicules particularly at mm-$lambda$ have been hampered by an absence of angular and temporal decision. With the provision of ALMA, that has now modified. ALMA can picture spicules with arcsec decision and might resolve their kinematics with excessive cadence imaging.
Right here, we briefly summarized ALMA observations of photo voltaic spicules in a polar coronal gap. Particulars could also be discovered within the lately printed paper by Bastian et al. (2025) or the arXiv reprint.
Determine 1. A comparability between ALMA and IRIS observations. Left: Spicules seen in 3 mm (decrease half) and by the IRIS slit-jaw imager within the 2796Å band (higher half). Proper: The identical, however the decrease a part of the composite reveals the 1.25 mm emission. The white arc signifies the photospheric limb.
ALMA Observations
The ALMA observations of photo voltaic spicules within the north polar coronal gap have been obtained on 2018 December 25 within the 3 mm band over a area of view of roughly 60” with a cadence of two s, and within the 1.25 mm band over the same area of view with a cadence of almost 2 min. The explanation for the gradual cadence of the 1.25 mm pictures is that mosaicking methods have been used to mix 14 discrete pointings at this wavelength to supply a area of view corresponding to that of the three mm band. As ALMA can not observe in two frequency bands concurrently, the three mm observations preceded those at 1.25 mm by about 3 hours.
We measured the variation of brightness temperature with top for an ensemble of spicules in each mm-$lambda$ bands. If the temperature of the spicular plasma is thought, the optical depth could be inferred as a operate of top, from which the column density could be inferred and, therefore, the electron quantity density $n_e$. We assumed two schematic fashions for the variation of spicule temperature with top: a) that it was isothermal; b) that the temperature elevated linearly to transition area values. It is very important observe that densities inferred from mm-$lambda$ emission are remarkably insensitive to the plasma temperature, with $n_e propto T_e^{1/4}$. No matter its temperature, the ALMA observations seize the density of spicular materials.
Determine 2: Spicule densities inferred from the mm-$lambda$ observations. Densities based mostly on 1.25 mm measurements are proven in blue and people based mostly on 3 mm measurements are proven in pink. a) Spicules are assumed to be isothermal with a temperature of $1.5times 10^4$ Okay. b) Spicules are assumed to have a temperature of $1.5times 10^4$ Okay at a top of two Mm, rising linearly to $10^5$ Okay at a top of 15 Mm. In each panels, the stable inexperienced line represents the match to the aggregated 1.25 and three mm information. The info factors are from Alissandrakis et al. (2018) (stuffed inexperienced circles), Beckers (1972) (purple squares), and from Krall et al. (1976) (orange diamonds). The dash-dot strains labeled “B68″, “B72″, and “A76″ are the densities ensuing from spicular filling components based mostly on the mannequin of Beckers (1968, 1972), and Athay (1976), respectively.
We discover that spicular densities derived from the dual-band mm-$lambda$ observations evaluate nicely with these inferred from historic O and UV observations. When the line-of-sight filling issue is taken into consideration, the variation of density with top is considerably flatter (dashed strains in Determine 2). We additionally discover that the upward mass flux on account of spicules falls quickly with top, calling into query whether or not they play a major function within the mass price range of the photo voltaic corona and photo voltaic wind. Nevertheless, we can not exclude the concept that electrical currents or wave modes carried by spicules could play a job in transporting vitality into the photo voltaic corona.
Conclusions
ALMA observations at mm-$lambda$ present new insights into quite a lot of photo voltaic phenomena, together with photo voltaic spicules. Our outcomes don’t present assist for the concept that spicules present important mass to the corona. However, extra work may be very a lot wanted: multi-band observations of spicules in quite a lot of environments (coronal holes, quiet Solar) in addition to detailed quantitative comparisons with observations made at O/UV/EUV wavelengths.
Primarily based on the latest paper by S. Bastian, C. Alissandrakis, A. Nindos, M. Shimojo, and S. M. White, ALMA Observations of Photo voltaic Spicules in a Polar Coronal Gap, ApJ 980 60 (2025). DOI:10.3847/1538-4357/ada445
References
Alissandrakis, C., et al. 2018, Photo voltaic Phys. 293, 20 doi: 10.1007/s11207-018-1242-4
Anthay, R. G., 1976 “The photo voltaic chromosphere and corona: Quiet solar”, Vol. 53, doi: 10.1007/978-94-010-1715-2
Bastian, T. S., Alissandrakis, C., Nindos, A., Shimojo, M., and White, S. M. 2025, ApJ 980, 60, doi: 10.3847/1538-4357/ada445.
Beckers, J. 1968 SoPh, 3, 367, doi: 10.1007/BF00171614
Beckers, J. 1972, ARA&A, 10, 73, doi: 10.1146/annurev.aa.10.090172.000445
Krall, Okay. R., Bessey, R. J., & Beckers, J. M. 1976, SoPh, 46, 93, doi: 10.1007/BF00157556
