H^{\ast}$ using various magnetic measurements of $H_{irr}^{ab} (T)$: (a) Hg-1223, (b) Hg-1234, (c) Hg-1245, and (d) La-112. In (b) and (d) dashed lines indicate $H_{c2}^{ab} (T)$ from TDO measurements. In (d) we also illustrate $H_{c2}^{c}$ for comparison. We note reasonable consistency among different experimental techniques, indicating strong suppression of $H^{\ast} \equiv H_{irr} ^{ab} (0)$ relative to $H_{c2} ^{ab} (0)$ (or $H_p$) in all cuprates.[]{data-label="fig2"}](Fig2){width="3.45in"}

  --------- --------- -------- -------- ------------------------- ------- -------- -------- -------- -------- ------------ ------- -------- --------
   Hg-1245   $0.15$    $1.30$   $0.80$    $55$ [@Hg1245comment]    $25$    $0.06$   $0.3$    $23.0$   $5.0$     $-[278]$    $40$    $0.08$   $0.02$
   Hg-1223   $0.15$    $1.04$   $0.92$       $52$ [@Zech96]        $18$    $0.26$   $0.9$    $48.5$   $6.5$     $-[347]$    $50$    $0.14$   $0.02$
   Hg-1234   $0.15$    $1.20$   $0.80$       $52$ [@Zech96]        $10$    $0.13$   $0.2 $   $75.0$   $10.0$    $-[320]$    $46$    $0.23$   $0.02$
   La-112    $0.10$    $1.00$   $1.00$       $13$ [@Zapf05]        $4.0$   $0.77$   $2.4 $   $46.0$   $4.0$    $160[110]$   $10$    $0.42$   $0.04$
   Bi-2212   $0.225$   $1.00$   $1.00$   $11$ [@Krusin-Elbaum04]   $8.0$   $2.05$   $15  $   $65.0$    $10$    $100[155]$   $22$    $0.42$   $0.06$
    NCCO     $0.15$    $1.00$   $1.00$        $13$ [@Yeh92]        $5.0$   $1.15$   $4.4 $   $40.0$   $5.0$     $77[59]$    $8.0$   $0.68$   $0.12$
    Y-123    $0.13$    $1.00$   $1.00$       $7.0$ [@Zech96]       $2.0$   $1.86$   $5.3 $   $210$     $50$    $600[239]$   $25$    $0.88$   $0.23$
  --------- --------- -------- -------- ------------------------- ------- -------- -------- -------- -------- ------------ ------- -------- --------

\[table1\]

The physical significance of $h^{\ast}$ may be better understood by considering how the magnetic irreversibility for $H \parallel ab$ occurs. For sufficiently low $T$ and small $H$, a supercur