Study of Hα macrospicules in coronal holes:: Magnetic structure and evolution in relation to photospheric magnetic setting

Small-scale solar dynamic events such as spicules, macrospicules, and microflares may play an important role in the coronal heating and solar wind acceleration in coronal holes. In these regions, the network fields concentrated along edges of supergranules are probably the source of the fine-scale activity that may drive the heating and acceleration. Recent H alpha limb observations from Big Bear Solar Observatory (BBSO) have shown that most macrospicules have two different forms of magnetic structure - a spiked jet or an erupting loop, suggesting two different formation mechanisms. In this paper, we analyze BBSO H alpha images and magnetograms of a coronal hole region near the disk center to study the evolution of the two types of macrospicule in relation to the magnetic arrangement at their base. We identified 78 macrospicules from the best day of 3 days of observations. Of these, 65 events were in the form of a spiked jet and were rooted in compact bipolar fields at the edges of the magnetic network. This supports the idea that spiky macrospicules are driven by reconnection between the network bipole and open magnetic fields. We also found five macrospicules that were in the form of an erupting loop oriented along a neutral line between the positive and negative network flux. They appear to be minifilament eruptions. Our results verify the magnetic structure inferred from our previous limb observations and support scenarios of coronal heating and solar wind generation through fine-scale explosive reconnection events seated in the magnetic network.