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Are cosmic rays effective for ionization of protoplanetary disks?The principal uncertainty in studying the magnetic properties of protoplanetary disks concerns the ionization levels of the disk's gas. The low gas temperature precludes thermal ionization, leaving cosmic rays as the dominant source of ionization. It has been shown that the resulting electrical conductivity is just high enough for a MHD dynamo to produce contemporaneously a magnetic field in most of the extended parts of a turbulent protoplanetary disk. Here we argue that the effectiveness of cosmic rays to ionize the bulk of the gas is impaired by the influence of the generated magnetic field on the propagation of cosmic rays within a disk. Cosmic rays scatter on magnetic inhomogeneities, and, as a result, their penetration depth decreases to only a fraction of the disk half-thickness, resulting in a severe depletion of free charge from the midplane regions of a disk. That, in turn, undercuts the dynamo mechanism, so extended parts of the disk are free from a dynamically significant magnetic field. We also point out that any additional, even small, in situ source of ionization, such as radioactive Al-26, may again make a dynamo a viable regeneration process capable of producing a dynamically important magnetic field.
Document ID
19950034148
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Dolginov, Arkady Z.
(Lunar and Planetary Inst. Houston, TX, United States)
Stepinski, Tomasz F.
(Lunar and Planetary Inst. Houston, TX, United States)
Date Acquired
August 16, 2013
Publication Date
May 20, 1994
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 427
Issue: 1
ISSN: 0004-637X
Subject Category
Astrophysics
Accession Number
95A65747
Funding Number(s)
CONTRACT_GRANT: NASW-4574
Distribution Limits
Public
Copyright
Other

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