Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...508..885f&link_type=abstract
The Astrophysical Journal, Volume 508, Issue 2, pp. 885-898.
Astronomy and Astrophysics
Astronomy
78
Sun: Activity, Sun: Corona, Sun: Magnetic Fields, Sun: X-Rays, Gamma Rays
Scientific paper
A comparison of X-ray images of the Sun and full disk magnetograms shows a correlation between the locations of the brightest X-ray emission and the locations of bipolar magnetic active regions. This correspondence has led to the generally accepted idea that magnetic fields play an essential role in heating the solar corona. To quantify the relationship between magnetic fields and coronal heating, the X-ray luminosity of many different active regions is compared with several global (integrated over entire active region) magnetic quantities. The X-ray measurements were made with the SXT Telescope on the Yohkoh spacecraft; magnetic measurements were made with the Haleakala Stokes Polarimeter at the University of Hawaii's Mees Solar Observatory. The combined data set consists of 333 vector magnetograms of active regions taken between 1991 and 1995; X-ray luminosities are derived from time averages of SXT full-frame desaturated (SFD) images of the given active region taken within +/-4 hours of each magnetogram. Global magnetic quantities include the total unsigned magnetic flux Phi_tot = ∫ dA|B_z|, B^2_z,tot= dAB^2_z, J_tot = ∫ dA|J_z|, and B^2_⊥,tot= dAB^2_⊥, where J_z is the vertical current density and B_z and B_⊥ are the vertical and horizontal magnetic field amplitudes, respectively. The X-ray luminosity L_X is highly correlated with all of the global magnetic variables, but it is best correlated with the total unsigned magnetic flux Phi_tot. The correlation observed between L_X and the other global magnetic variables can be explained entirely by the observed relationship between those variables and Phi_tot. In particular, no evidence is found that coronal heating is affected by the current variable J_tot once the observed relationship between L_X and Phi_tot is accounted for. A fit between L_X and Phi_tot yields the relationship L_X ~= 1.2 x 10^26 ergs s^-1(Phi_tot/10^22 Mx)^1.19. The observed X-ray luminosities are compared with the behavior predicted by several different coronal heating theories. The Alfvén wave heating model predicts a best relationship between L_X and Phi_tot, similar to what is found, but the observed relationship implies a heating rate greater than the model can accommodate. The ``Nanoflare Model'' of Parker predicts a best relationship between L_X and B^2_z,tot rather than Phi_tot, but the level of heating predicted by the model can still be compared to the observed data. The result is that for a widely used choice of the model parameters, the nanoflare model predicts 1.5 orders of magnitude more heating than is observed. The ``Minimum Current Corona'' model of Longcope predicts a qualitative variation of L_X with Phi_tot that agrees with what is observed, but the model makes no quantitative prediction that can be tested with the data. A comparison between L_X and the magnetic energy E_mag in each active region leads to a timescale that is typically 1 month, or about the lifetime of an active region, placing an important observational constraint on coronal heating models. Comparing the behavior of solar active regions with nearby active stars suggests that the relationship observed between L_X and Phi_tot may be a fundamental one that applies over a much wider range of conditions than is seen on the Sun.
Fisher George H.
Longcope Dana Warfield
Metcalf Thomas R.
Pevtsov Alexei A.
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